Tapered multilayer luminaire devices

ABSTRACT

An optical device for collecting light and selectively outputting or concentrating the light. A wedge layer has an optical index of refraction n 1 , and top, bottom and side surfaces intersecting to define an angle of inclination φ. A back surface spans the top, bottom and side surface. A first layer is coupled to the bottom surface of the layer and has an index of refraction n 2 . The first layer index n 2  causes light input through the back surface of the layer to be preferentially output into the first layer. A second layer is coupled to the bottom of the first layer and selectively causes output of light into ambient. Additional layers, such as a light polarization layer, a polarization converting layer and a post LCD diffuser layer can be used to make preferential use of polarized light or diffuse light having passed through the LCD layer to enhance viewing of the output light.

This is a continuation in part of Ser. No. 08/029,883 filed on Mar. 11,1993, now U.S. Pat. No. 5,303,322 and a continuation in part of parentcase Ser. No. 07/855,838 filed on Mar. 23, 1992, now U.S. Pat. No.5,237,641.

The present invention is concerned generally with a luminaire device forproviding selected light illumination. More particularly, the inventionis concerned with tapered luminaires, such as a wedge or disc shape, forbacklighting by selective post diffusion of light output from a liquidcrystal display layer and also be manipulating light polarization andfiltering selected light polarization to enhance light illumination andimage output.

A variety of applications exist for luminaire devices, such as, forliquid crystal displays. For flat panel liquid crystal displays, it isimportant to provide adequate backlighting while maintaining a compactlighting source. It is known to use wedge shaped optical devices forgeneral illumination purposes. Light is input to such devices at thelarger end; and light is then internally reflected off the wedgesurfaces until the critical angle of the reflecting interface isreached, after which light is output from the wedge device. Suchdevices, however, have only been used to generally deliver anuncollimated lighting output and often have undesirable spatial andangular output distributions. For example, some of these devices usewhite painted layers as diffuse reflectors to generate uncollimatedoutput light.

It is therefore an object of the invention to provide an improvedoptical device and method of manufacture.

It is another object of the invention to provide a novel threedimensional luminaire.

It is a further object of the invention to provide an improvedmultilayer tapered luminaire for optical purposes, such as forcontrolled utilization of light polarization.

It is still another object of the invention to provide a novel taperedluminaire device for controlled transmission or concentration of light.

It is an additional object of the invention to provide a novel opticaldevice for providing collimated polarized light illumination from thedevice.

It is yet a further object of the invention to provide an improvedtapered luminaire having a polarization filter layer.

It is still another object of the invention to provide a novel luminaireallowing conversion of polarized light to enhance illumination outputfrom the invention.

It is yet a further object of the invention to provide an improvedillumination system wherein a combination of a polarization filter layerand light redirecting layer are utilized to provide improved lightillumination over a controlled angular range of output to the viewer.

It is still a further object of the invention to provide a novelluminaire optical device wherein a combination of a polarization filter,polarization converting layer and a post LCD diffuser layer are used toenhance light illumination from the optical device.

It is yet a further object of the invention to provide an improvedluminaire optical device wherein an LCD layer is disposed adjacent anoverlying post LCD diffuser layer to enable control of lightdistribution over broader angles to viewers without loss of light outputor image qualifies.

Other objects, features and advantages of the present invention will bereadily apparent from the following description of the preferredembodiments thereof, taken in conjunction with the accompanying drawingsdescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art wedge shaped device;

FIG. 2A illustrates a multilayer tapered luminaire device constructed inaccordance with the invention; FIG. 2B is a magnified partial view ofthe junction of the wedge layer, the first layer and the second facetedlayer; FIG. 2C is an exaggerated form of FIG. 2A showing a greatlyenlarged second faceted layer; FIG. 2D is a partial view of the junctionof the three layers illustrating the geometry for brightnessdeterminations; FIG. 2E is a multilayer wedge device with a lightredirecting, internally transmitting layer on the bottom; FIG. 2F showsa wedge device with a lower surface translucent layer; FIG. 2G shows awedge layer with a lower surface refracting faceted layer; FIG. 2H showsa wedge layer with a lower surface refracting layer and curved facetsthereon; FIG. 2I shows a wedge layer with a refracting layer of facetshaving variable facet angles; FIG. 2J shows a single refracting prismcoupled to a wedge layer; FIG. 2K shows a single refracting prismcoupled to a wedge layer and with an integral lens; FIG. 2L shows areflecting faceted layer coupled to a wedge device; FIG. 2M shows areflecting faceted layer with curved facet angles and coupled to a wedgedevice; FIG. 2N shows a flat reflecting facet on a wedge layer and FIG.20 shows a curved reflecting facet on a wedge layer;

FIG. 3A illustrates a multilayer wedge device with curved facets on theambient side of the second layer and FIG. 3B shows a magnified partialview of the junction of the various layers of the device;

FIG. 4A shows calculated brightness performance over angle for anasymmetric range of angles of illumination; FIG. 4B shows calculatedbrightness distribution performance over angle for a more symmetricangle range; FIG. 4C illustrates calculated brightness performance overangle for the symmetry of FIG. 4B and adding an external diffuserelement; FIG. 4D illustrates an output using flat reflecting facets, noparallel diffuser; full-width at half-maximum brightness (FWHM)=7degrees; FIG. 4E illustrates an example of nearly symmetrical outputdistribution; measured using flat facets with parallel lenticulardiffuser; FWHM=34 degrees; FIG. 4F illustrates an example ofasymmetrical output distribution, measured using curved facets; FWHM=32degrees; FIG. 4G illustrates an example asymmetrical outputdistribution, measured using curved facets; FWHM=26 degrees; FIG. 4Hillustrates an example of a bimodal output distribution, measured usingone faceted reflecting layer and one faceted refractive layer; andFIG.4I illustrates an example of an output distribution with large"tails", measured using a diffuse reflective bottom redirecting layerand a refracting/internally-reflecting top redirecting layer;

FIG. 5A shows a top view of a disc shaped light guide and FIG. 5Billustrates a cross section taken along 5B--5B in FIG. 5A;

FIG. 6A shows a cross sectional view of a multilayer tapered luminairedevice with an air gap layer included; FIG. 6B shows another taperedluminaire in cross section with a compound parabolic lightsource/concentrator; FIG. 6C illustrates another tapered luminaire incross section with a variable parametric profile light source and alenticular diffuser; and FIG. 6D shows another tapered luminaire incross section with non-monotonic wedge layer thickness;

FIG. 7 illustrates a reflective element disposed concentrically about alight source;

FIG. 8 illustrates a reflective element disposed about a light sourcewith maximum displacement between the reflector center of curvature andthe center of the light source;

FIG. 9A illustrates use of a redirecting layer to provide asubstantially similar angular distribution emanating from all portionsof the device and FIG. 9B illustrates use of a redirecting layer to varyangular distribution emanating from different portions of the device,and specifically to focus the various angular distributions to enhancetheir overlap at a selected target distance;

FIG. 10 illustrates one form of pair of lenticular arrays of aluminaire; and

FIG. 11 illustrates a lenticular diffuser array and curved facet layerof a luminaire;

FIG. 12A illustrates a wedge shaped luminaire having a pair ofdiffraction gratings or hologram layers; FIG. 12B shows a wedge shapedluminaire with a pair of refracting facet layers and diffusers; FIG. 12Cillustrates a wedge shaped luminaire with a pair of faceted layers; FIG.12D shows a wedge shaped luminaire with two refracting single facetlayers; FIG. 12E illustrates a wedge shaped luminaire with a refractingsingle facet layer and a bottom surface redirecting layer; FIG. 12Fshows a luminaire with a top surface redirecting layer of a refractingfaceted layer and a bottom surface refracting and internally reflectinglayer; FIG. 12G illustrates a luminaire with a top surfacerefracting/internally reflecting faceted layer and a bottom surfacerefracting/internally reflecting faceted layer; FIG. 12H shows aluminaire with a top surface refracting faceted layer and a bottomsurface refracting/internally reflecting faceted layer; FIG. 12Iillustrates a luminaire with a bottom surface specular reflector and atop layer transmission diffraction grating or transmission hologram;FIG. 12J shows a luminaire with a bottom surface specular reflector anda top surface refracting faceted layer and diffuser; FIG. 12Killustrates a luminaire with a bottom layer specular reflector and a toplayer refracting/internally reflecting faceted layer; FIG. 12L shows aluminaire with a bottom specular reflector and a top layerrefracting/internally reflecting faceted layer; FIG. 12M illustrates aluminaire with an initial reflector section including an integrallenticular diffuser; FIG. 12N shows a luminaire with a roughened initialreflector section of a layer; FIG. 12O illustrates a luminaire with aneccentric light coupler and converging to the wedge shaped section; FIG.12P shows a luminaire with an eccentric light coupler and a diffuser androughened or lenticular reflector; FIG. 12Q illustrates a luminaire witha bottom specular or diffusely reflecting layer and a top refractinglayer and FIG. 12R shows a luminaire for generating a "bat wing" lightoutput;

FIG. 13 illustrates a combination of two wedge shaped sections formedintegrally and using two light sources;

FIG. 14 shows a tapered disk luminaire including a faceted redirectinglayer;

FIG. 15 illustrates a luminaire operating to provide a collimated lightoutput distribution;

FIG. 16A shows a prior art ambient mode LCD and FIG. 16B illustrates aprior art transflective LCD unit;

FIG. 17 shows a luminaire operative in ambient and active modes with afaceted redirecting layer and a lenticular diffuser; and

FIG. 18A illustrates a luminaire with an array of microprisms for afaceted surface disposed over a diffuse backlight and with themicroprisms having equal angles on both sides, but each microprismhaving progressively changing facet angles across the face; FIG. 18Bshows a microprism array as in FIG. 18A with the sides of eachmicroprism having different angles varying again across the facetedsurface;

FIG. 19A illustrates a luminaire having a polarization filter layer;FIG. 19B shows a luminaire with a plurality of layers including apolarization filter layer; and FIG. 19C shows a variation on FIG. 19Bwith layer indices enabling output of both polarizations of light on oneside of the luminaire;

FIG. 20A illustrates a luminaire similar to FIG. 19B but furtherincludes a reflector layer; FIG. 20B illustrates a luminaire as in FIG.20A but a redirecting layer is disposed on the same side of the baselayer and the polarization filter; and FIG. 20C is a variation on FIG.20B with an additional redirecting layer and rearranged n₂/filter/redirecting layers;

FIG. 21A illustrates a luminaire having a polarization converting layerand polarization filter layer; FIG. 21B is a variation on FIG. 21A withthe polarization filter layer and polarization converting layer on thesame side of the base layer;

FIG. 22A illustrates a luminaire with a polarization filter layer oneside of the base layer and a polarization converting layer on the otherside; FIG. 22B shows a variation on FIG. 22A with the filter andconverting layers adjacent one another on the same side of the baselayer; FIG. 22C shows a further variation of FIG. 22A and B and with areflector layer added; FIG. 22D illustrates a further variation on FIG.22C with the converting layer moved to the other side of the base layerand FIG. 22E shows another variation on FIG. 22D;

FIG. 23A illustrates a luminaire having plural layers including apolarization filter, a converting, a redirecting, a reflector and an LCDlayer; FIG. 23B shows a variation on FIG. 23A; and FIG. 23C illustratesyet another variation on FIG. 23A;

FIG. 24A illustrates conventionally a luminaire with two polarizationfilter layers for two polarization states; FIG. 24B shows a variation onFIG. 24A plus an added light redirecting layer; FIG. 24C is a furthervariation on FIG. 24B with a matching layer, a second redirecting layerand an LCD layer; FIG. 24D is yet another variation on FIGS. 24B and C;FIG. 24E is a variation on FIG. 24D with an added converting layer andtwo polarization filter layers and two redirecting layers and FIG. 24Fis still another variation on FIG. 24E with LCD layers on both sides ofthe base layer;

FIG. 25A illustrates a general construction utilizing two polarizationfilter layers and a polarization converting layer; FIG. 25B shows avariation on FIG. 25A with an added redirecting layer;

FIG. 26A illustrates a multilayer luminaire with a light source coupledto a light angle transformer to control spatial uniformity of lightouput from the device; FIG. 26B is a variation on FIG. 26A;

FIG. 27A illustrates a luminaire with a faceted redirecting layer andlight polarization and polarization converting layers; and FIG. 27B is avariation on FIG. 27A, wherein the redirecting layers includes areflecting layer with curved facets for focusing light in a preferredviewing zone;

FIG. 28A illustrates a luminaire including a polarization light filter,polarization converter and a faceted redirecting and diffusing layer;FIG. 28B shows a variation on FIG. 28A with two polarization filterlayers and two faceted redirecting layer; FIG. 28C shows a light sourcecoupled to a luminaire and is a variation on FIG. 28A; FIG. 28D is avariation on FIG. 28C; and FIG. 28E is yet another variation on FIG.28C;

FIG. 29A illustrates a luminaire with polarized light output incombination with an LCD layer and FIG. 29B is a variation on FIG. 29A;

FIG. 30A illustrates a conventional LCD display system; FIG. 30B shows apolarization filter layer; FIG. 30C illustrates a multilayer thin filmform of polarization filter; FIG. 30D shows a Brewster Stack form ofpolarization filter; FIG. 30E illustrates a birefringent plate andinteracting polarized light; FIG. 30F shows Eulerian angles and opticalvectors; FIG. 30G shows a backlight providing collimated light in the xzplane and FIG. 30H shows a detailed enlargement of a zone from FIG. 30G;

FIG. 31A illustrates a luminaire with a coupled birefringent layer; FIG.31B shows a luminaire and birefringent layer and an added lightredirecting layer; FIG. 31C illustrates a luminaire system similar toFIG. 31B with an added light polarization converting layer; FIG. 31D issimilar to FIG. 31C but the converting layer is on the same side of thebase layer as the birefringent layer; FIG. 31E illustrates a variationon FIG. 31C with the converting layer coupled directly to the baselayer; FIG. 31F is similar to FIG. 31D but the redirecting layercomprises a faceted layer; FIG. 31G is based on the embodiment of FIG.31F but also includes a matching layer, an LCD layer and a diffuserlayer; and FIG. 31H is a variation on FIG. 31G; and

FIG. 32A illustrates a luminaire system including an LCD layer and apost LCD diffuser layer for processing unpolarized light; FIG. 32B is avariation on FIG. 32A; and FIG. 32C is a variation on FIG. 32B.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A multilayer luminaire device constructed in accordance with one form ofthe invention is illustrated in FIG. 2 and indicated generally at 10. Aprior art wedge 11 is shown generally in FIG. 1. In this wedge 11 thelight rays within the wedge 11 reflect from the surfaces until the angleof incidence is less than the critical angle (sin⁻¹ 1/n) where n is theindex of refraction of the wedge 11. The light can exit equally fromboth top and bottom surfaces of the wedge 11, as well as exiting atgrazing angles.

The multilayer luminaire device 10 (hereinafter "device 10") shown inFIG. 2A includes base or wedge layer 12 which has a characteristicoptical index of refraction of n₁. The term "wedge layer" shall be usedherein to include all geometries having converging top and bottomsurfaces with wedge shaped cross sectional areas. The x, y and z axesare indicated within FIGS. 2A and 2C with the "y" axis perpendicular tothe paper. Typical useful materials for the wedge layer 12 includealmost any transparent material, such as glass, polymethyl methacrylate,polystyrene, polycarbonate, polyvinyl chloride, methylmethacrylate/styrene copolymer (NAS) and styrene/acrylonitrile. Thewedge layer 12 in FIG. 2A further includes a top surface 14, a bottomsurface 16, side surfaces 18, edge 26 and a back surface 20 of thicknessto spanning the top, bottom and side surfaces. A light source, such as atubular fluorescent light 22, injects light 24 through the back surface20 into the wedge layer 12. The light 24 is internally reflected fromthe various wedge layer surfaces and is directed along the wedge layer12 toward the edge 26. Other possible light sources can be used and willbe described hereinafter. Generally, conventional light sources providesubstantially incoherent, uncollimated light; but coherent, collimatedlight can also be processed by the inventions herein.

For the case where the surfaces 14 and 16 are flat, a single angle ofinclination φ for a linear wedge is defined by the top surface 14 andthe bottom surface 16. In the case of nonlinear wedges, a continuum ofangles φ are definable; and the nonlinear wedge can be designed toprovide the desired control of light output or concentration. Such anonlinear wedge will be described in more detail later.

In the embodiment of FIG. 2A a first layer 28 is coupled to the wedgelayer 12 without any intervening air gap, and the first layer 28 has anoptical index of refraction n₂ and is optically coupled to the bottomsurface 16. The first layer 28 can range in thickness from a few lightwavelengths to much greater thicknesses and accomplish the desiredfunctionality. The resulting dielectric interface between the wedgelayer 12 and the first layer 28 has a higher critical angle than at theinterface between the wedge layer 12 and ambient. As will be apparenthereinafter, this feature can enable preferential angular output andcollimation of the light 24 from the device 10.

Coupled to the first layer 28 is a second layer 30 (best seen in FIG.2B) having an optical index of refraction n₃ which is greater than n₂,and in some embodiments preferably greater than n₁. This configurationthen allows the light 24 to leave the first layer 28 and enter thesecond layer 30. In the embodiment of FIG. 2A there are substantially nointervening air gaps between the first layer 28 and the second layer 30.In the preferred form of the invention illustrated in FIG. 2A, n₁ isabout 1.5, n₂ <1.5 and n₃ ≧n₁. Most preferably, n₁ =1.5, n₂ <1.5 (suchas about one) and n₃ ≧n₁.

In such a multilayer configuration for the device 10 shown in FIG. 2,the wedge layer 12 causes the angle of incidence for each cyclic time ofreflection from the top surface 14 to decrease by the angle ofinclination 2φ (relative to the normal to the plane of the bottomsurface 16). When the angle of incidence with the bottom surface 16 isless than the critical angle characteristic of the interface between thewedge layer 12 and the first layer 28, the light 24 is coupled into thefirst layer 28. Therefore, the first layer 28 and the associated opticalinterface properties form an angular filter allowing the light 24 topass when the condition is satisfied: θ<θ_(c) =sin⁻¹ (n₂ /n₁). That is,the described critical angle is higher than for the interface betweenair and the wedge layer 12. Therefore, if the two critical angles differby more than 6φ, nearly all of the light 24 will cross into theinterface between the wedge layer 12 and the first layer 28 before itcan exit the wedge layer 12 through the top surface 14. Consequently, ifthe two critical angles differ by less than φ, a substantial fraction,but less than half, of the light can exit the top surface 14. If the twoangles differ by more than φ and less than 6φ, then substantially morethan half but less than all the light will cross into the wedge layer 12and the first layer 28 before it can exit the wedge layer 12 through thetop surface 14. The device 10 can thus be constructed such that thecondition θ<θ_(c) is satisfied first for the bottom surface 16. Theescaping light 24 (light which has entered the layer 28) will then enterthe second layer 30 as long as n₃ >n₂, for example. The light 24 thenbecomes a collimated light 25 in the second layer 30 provided by virtueof the first layer 28 being coupled to the wedge layer 12 and having theproper relationship between the indices of refraction.

In order to generate an output of the light 24 from the device 10, thesecond layer 30 includes means for scattering light, such as a paintlayer 33 shown in FIG. 2E or a faceted surface 34 shown in both FIGS. 2Band 2C. The paint layer 33 can be used to preferentially project animage or other visual information. The paint layer 33 can comprise, forexample, a controllable distribution of particles having characteristicindices of refraction.

By appropriate choice, light can also be redirected back through thewedge layer 12 and into ambient (see light 29 in FIGS. 2A and 2C) oroutput directly into ambient from the second layer 30 (see light 29' inFIG. 2F).

In other forms of the invention a further plurality of layers withassociated "n" values can exist. In one preferred form of the inventionthe index of the lowest index layer can replace n₂ in equations fornumerical aperture and output angle (to be provided hereinafter). Suchfurther layers can, for example, be intervening between the wedge layer12 and the first layer 28, intervening between the first layer 28 andthe second layer 30 or be overlayers of the wedge layer 12 or the secondlayer 30.

In certain embodiments the preferred geometries result in output oflight into ambient without being reflected back through the wedge layer12. For example, in FIG. 2F the device 10 can include a translucentlayer 37. In another form of this embodiment shown in FIG. 2G, arefracting layer 38 is shown. The refracting layer 38 can include flatfacets 39 for providing a collimated output. Also shown in phantom inFIG. 2G is a transverse lenticular diffuser 83 which will be describedin more detail hereinafter. The diffuser layer 83 can be used with anyof the invention geometries, including above the wedge layer 12 as inFIG. 6A.

In yet another example shown in FIG. 2H, the refracting layer 38 caninclude curved facets 41 for providing a smoothly broadened output overa desired angular distribution. In a further example shown in FIG. 21,the refracting layer 38 includes variable angle facets 42. These facets42 have facet angles and/or curvature which are varied with positionacross the facet array to focus output light in a desired manner. Curvedfacets would enable producing a softly focused region within which theentire viewing screen appears to be illuminated. Examples of theapplication to computer screen illumination will be describedhereinafter. In FIGS. 2J and 2K are shown, respectively, a singlerefracting prism element 43 and the prism element 43 with an integrallens 44 to focus the output light. FIGS. 2L and M show the facetedsurface 34 with the facets angularly disposed to control the outputdistribution of light. In FIGS. 2K and 2L the light is output to a focalpoint "F", while in FIG. 2M the output is over an approximate viewingrange 45. FIGS. 2N and 20 illustrate flat reflecting facets 48 andcurved reflecting facet 49 for providing a collimated light output orfocused light output, respectively.

As shown in FIGS. 2A and C the faceted surface 34 optically reflects andredirects light 29 through the second layer 30, the first layer 28 andthen through the wedge layer 12 into ambient. Only a fraction of eachfacet is illuminated, causing the output to appear alternately light anddark when viewed on a sufficiently small scale. Since this pattern istypically undesirable, for the preferred embodiment shown in FIG. 2B theperiod of spacing between each of the faceted surfaces 34 is preferablylarge enough to avoid diffraction effects, but small enough that theindividual facets are not detected by the intended observing means. Thespacing is also chosen to avoid forming Moire interference patterns withany features of the device to be illuminated, such as a liquid crystaldisplay or CCD (charge coupled device) arrays. Some irregularity in thespacing can mitigate undesirable diffraction Moire effects. For typicalbacklighting displays, a spacing period of roughly 0.001-0.003 inchescan accomplish the desired purpose.

The faceted surface 34 in FIGS. 2B and 2C, for example, can be generallyprepared to control the angular range over which the redirected light 29is output from the device 10. The minimum distribution of output anglein the layer 30 has a width which is approximately equal to:

    Δθ=2φ[(n.sub.1.sup.2 -n.sub.2.sup.2)/(n.sub.3.sup.2 -n.sub.2.sup.2)].sup.1/2

Thus, since φ can be quite small, the device 10 can be quite aneffective collimator. Therefore, for the linear faceted surface 34, theexiting redirected light 29 has a minimum angular width in air ofapproximately:

    Δθair=n.sub.3 Δθ=2φ(n.sub.1.sup.2 -n.sub.2.sup.2)/[1-(n.sub.2 /n.sub.3).sup.2 ].sup.1/2.

As described hereinbefore, and as shown in FIGS. 2H, 21, 2K, 2L, 2M, andFIG. 3, the facet geometry can be used to control angular output inexcess of the minimum angle and also focus and control the direction ofthe output light.

Fresnel reflections from the various interfaces can also broaden theoutput angle beyond the values given above, but this effect can bereduced by applying an anti reflection coating 31 on one or more of theinternal interfaces, as shown in FIG. 2B.

The brightness ratio ("BR") for the illustrated embodiment can bedetermined by reference to FIG. 2D as well as by etendue match, and BRcan be expressed as: ##EQU1## or, B.R.=illuminated area/total area

B.R.=[1-(n₂ /n₃)² ]1/2=0.4-0.65 (for most transparent dielectricmaterials). For example, the wedge layer 12 can be acrylic (n₁ =1.49),the first layer 28 can be a fluoropolymer (n₂ =1.28-1.43) or Sol-gel (n₂=1.05-1.35, fluoride salts (n₂ =1.38-1.43) or silicone based polymer oradhesive (n₂ =1.4-1.45); and the second layer 30 can be a facetedreflector such as polycarbonate (n₃ =1.59), polystyrene (n₃ =1.59) epoxy(n₃ =1.5-1.55) or acrylic (n₃ =1.49) which have been metallized at theair interface.

The flat, or linear, faceted surfaces 34 shown, for example, in FIGS. 2Band 2C can redirect the incident light 24 to control direction of lightoutput and also substantially preserve the angular distribution of lightΔθ which is coupled into the second layer 30 by the angle-filteringeffect (see, for example, FIG. 4D). For example, in one preferredembodiment shown in FIG. 2L, the faceted surfaces 34 reflect light withthe flat facet angles varied with position to focus the output light. InFIG. 2M the faceted surfaces 34 include curved facet angles which varywith position to produce a softly focused viewing zone 45 within whichthe entire screen appears to be illuminated (see also, for example FIGS.4F and 4G). Also show in phantom in FIG. 2M is an exemplary liquidcrystal display 47 usable in conjunction with the invention. As furthershown in FIGS. 3A and B, curved facets 36 also redirect the incidentlight 24, but the facet curvature increases the resulting range ofangular output for the redirected light 29 (see for comparison for flatfacets FIG. 2D). For example, it is known that a concave trough canproduce a real image, and that a convex trough can produce a virtualimage (see, for example, FIG. 3B). In each case the image is equivalentto a line source emitting light uniformly over the desired angularoutput range. Consequently, an array of such trough shaped facets 36 canredirect the incoming form of collimated light 25 from the first layer28 (see FIG. 2C), and a plurality of such line source images then formthe redirected light 29. By arranging the spacing of the curved facets36 to less than human eye resolution, the resulting array of linesources will appear very uniform to an observer. As previouslymentioned, the choice of about three hundred to five hundred lines/inchor 0.002 to 0.003 inches for the period of facet spacing provides such aresult. For a typical LCD display viewing distances of approximatelytwenty inches or greater are conventional.

Other useful facet shapes can include, for example, parabolic,elliptical, hyperbolic, circular, exponential, polynomial, polygonal,and combinations thereof. The user can thus construct virtuallyarbitrary distributions of averaged brightness of illumination usingdifferent facet designs. For example, polygon shaped facets can be usedto produce output angular distributions having multiple peaks.

Examples of brightness distribution over various ranges of angularoutput using a curved-faceted reflector are illustrated in FIGS. 4A-4C,4F and 4G. FIG. 4C and 4E shows the brightness distribution in the caseof a reflector having linear facets, and further including a diffuserelement 40 (shown in phantom in FIG. 2C). The predicted performanceoutput is shown for the various angular ranges (see FIGS. 4A-4C) andcompared with the measured angular output of light for a commerciallyavailable source (labeled "Wedge"), such as a "Wedge Light" unit, atrademark of Display Engineering. The preferred angular range canreadily be modified to accommodate any particular viewing andcollimation requirements up to the minimum angle Δθ (air) describedhereinbefore by the equation in terms φ, n₁, n₂ and n₃. Thismodification can be accomplished by progressively changing the curvatureof the curved facets 36 in the manner shown in FIG. 2M and discussedhereinbefore. In addition to the illustrated control of the verticalviewing angular range, modification of the horizontal viewing range canalso be accomplished by appropriate changes of the shape of the curvedfacets 36. The above described angular distributions shown in FIGS.4A-4I are representative when the device 10 is processing the light 24within the numerical aperture NA=(n₁ ² -n₂ ²)^(1/2). When light isoutside this range, additional techniques can be applied to help controlthe angular output range.

FIGS. 9A and 9B further illustrate the use of redirecting means toprovide a tightly overlapping focused illumination output and a lessoverlapping focused illumination output, respectively. These conceptscan be applied practically by considering that a typical portablecomputer screen 87 has a vertical extent "V" of about 150 mm, while atypical viewing distance, "D", is 500 mm. A viewer at distance "D",positioned normal to the vertical center of the computer screen 87 willview different areas of the screen 87 at angles ranging from -8.5°measured at the top of the screen 87 to +8.5° measured at the bottom ofthe screen 87. This variation in viewing angle can, however, causeundesirable effects in use of a system having such screen illumination.Such a limited illumination angle for the screen 87 implies a limitedrange of positions from which a viewer can see a fully illuminatedscreen 87 (see FIG. 9A). Defining the viewer position in terms of theangle and distance from the center of the screen 87, then the effectiveangular range is substantially reduced below the nominal illuminationangle. For example, if the nominal illumination range is ±20° measuredat each individual facet, then the effective viewing range is reduced to±12° in the typical flat panel illuminator shown in FIG. 9A. Theresulting illumination between 12°-20°, either side of center for thescreen 87, will appear to be nonuniform to the viewer.

The invention herein can be used to overcome the above describednonuniformities by controlling the orientation of the faceted surface34. As illustrated, for example, in FIG. 2M both surfaces of the facetsare rotated progressively such that the flat facet surface is variedfrom 35.6° to 33.3° relative to, or parallel to, the edges of the planesdefining the various layers of the device 10. This systematic variationfrom the top to the bottom of screen 89 (see FIG. 9B) results in theredirected output illustrated. The faceted surface 34 can further becombined with the diffuser 83 and the like to produce a variable, butcontrollable light illumination output distribution. A flat facetedsurface 168 can further be combined with a diffuser 170. Therefore, asshown in FIG. 9B the ability to rotate the angular distributions oflight at different points on the screen 89 enable compensation for thevariation in viewing angle with position. Systematic variations in thefaceted surface 34 can further include variations in one or more facetangles, the spacing of the facets 38, or the depth and width of theindividual facets 38. In other embodiments, the same principles can beapplied to focus the output of any faceted redirecting layer. Examplesare shown in FIGS. 2I and 2L.

In another example of overcoming nonuniformities of illumination, anarray of micro-prisms for the faceted surface 34 can be laid over aconventional diffuse backlight 101 (see FIG. 18A). This faceted surface34 operates by a combination of refraction and total internal reflectionto permit only a limited angular range to be output through the layerinto ambient. This angular range depends on the facet angles. For thecase of acrylic film (n=1.49), highest brightness is typically achievedwith a prism included angle of 90-100 degrees, resulting in a viewingangle of approximately ±35 degrees. Backlights using such a geometryshow a sharp "curtaining" effect which is disconcerting to many viewers.This effect can be ameliorated by rotating the facets 38 from top tobottom of the screen to produce a focusing effect (see FIG. 18B). Simpleray-tracing shows that, for included angles in the range of 100°-110°, afacet rotated by an angle θ will produce an angular distribution rotatedby approximately θ/2. In the embodiment shown in FIG. 18 the progressivevariation of facet face angle can vary as position χ along the facetedsurface 34 wherein, for example:

    ψ.sub.1 =35°-(0.133°/mm)·x

    ψ.sub.2 =35°+(0.133°/mm)·x

This progressive facet angle change will produce an angular distributionwhich varies by approximately ten degrees across the screen 89, andsatisfies the generic constraints outlined above.

Whatever the desired facet shapes, the faceted surface 34 (see, FIG. 2D)is preferably formed by a conventional process such as molding or otherknown milling processes. Details of manufacture will be describedhereinafter:

Nonlinear Wedges

In another form of the invention the wedge layer 12, which is theprimary lightguide, can be other than the linear shape assumedhereinbefore. These shapes allow achievement of a wide variety ofselected light distributions. Other shapes can be more generallydescribed in terms of the thickness of the wedge layer 12 as a functionof the wedge axis "z" shown in FIGS. 2B and C (the coordinate axis whichruns from the light input edge to the small or sharp edge 26). For thelinear shaped wedge,

    A(z)=A.sub.0 -C·z                                 (1)

A₀ =maximum wedge thickness (see FIG. 2A)

C=constant=tan φ

A large range of desired spatial and angular distributions can beachieved for the light output power (power coupled to the second layer30). This light output power is thus the light available for output tothe ambient by the appropriately faceted surfaces 34 or 36, or even bythe diffuse reflector 33 (see FIG. 2E) or other means.

For example, if L and M are direction cosins along the x and y axes,respectively, then L₀ and M₀ are the values of L and M at the thick edge(z=0). This initial distribution is Lambertian within some well-definedangular range, with little or no light outside that range. Thisdistribution is especially important because ideal non-imaging opticalelements have limited Lambertian output distributions. The keyrelationship is the adiabatic invariant, A(z)cos(θ_(c)) which isapproximately equal to A₀ L₀ and which implicitly gives the position (z)of escape. To illustrate this concept, suppose we desire uniformirradiance so that dP/dz=constant. Suppose further that the initialphase space uniformly fills an elliptical area described by thefollowing expression:

    L.sub.0.sup.2 /σ.sup.2 +M.sub.0.sup.2 /τ.sup.2 =1(2)

where τ is the dimension of an ellipse along the M axis and σ is thedimension of the ellipse along the L axis.

Then, dP/dL=const·[1-L² /σ² ]^(1/2) but dA/dz=[A₀ /L_(c) ]dL₀ /dZ whereL_(c) =cos θ_(c). Therefore, [1-(L_(c) A)² /(A₀ σ)² ]^(1/2) dA=constanttimes dz. Suppose σ=L_(c) in the preferred embodiment. This result canbe interpreted by the substitution A/A₀ sin u, so that A=A₀ sin u andu+1/2sin(2u)=(π/2)(1-z/D) where D is the length of the wedge layer 12.

If the desired power per unit length is dP/dz, more generally, then thedesired shape of the wedge layer 12 is determined by the differentialequation: ##EQU2##

Note that in all these cases the output distribution has onlyapproximately the desired form because it is modified by Fresnelreflections. Note also that even when the wedge device 10 is curved, ifthe curvature is not too large, it may still be useful to define anaverage angle φ which qualitatively characterizes the system.

In another aspect of the invention the geometry of the above exampleshas an x,y interface between two refractive media with indices n₁ andn₂. The components nM,nN are conserved across the interface so that n₁M₁ =n₂ M₂, n₁ N₁ =n₂ M₂. The angle of incidence projected in the x,zplane is given by sin θ_(eff) =N/(L² -N²)^(1/2). Then using the aboverelations, sin θ_(2eff) /sin θ_(1eff) =(n₁ /n₂)[1-M₁ ² ]^(1/2) /[1-(n₁/n₂)² M₁ ² ]^(1/2) =(n₁ /n₂)_(eff). For example, for n₁ =1.49, n₂ =1.35,M₁ =0.5, the effective index ratio is 1.035(n₁ /n₂), which is onlyslightly larger than the actual index ratio.

Variation of Index of Refraction Over Spatial Parameters

In the general case of tapered light guides, the wedge layer 12 isgenerally along the z axis with the narrow dimension along the x axis(see, for example, FIG. 2A). If we introduce optical direction cosines(nL,nM,nM) where L,M,N are geometric direction cosines along x,y,z, thenn is the refractive index which may vary with spatial position. Forguided rays in the wedge layer 12, the motion in x is almost periodic,and the quantity φnLdx for one period is almost constant as the raypropagates along z. This property is called adiabatic invariance andprovides a useful framework for analyzing the lightguide properties.

In a first example the wedge device 10 shown in FIG. 2A has a uniformindex in the wedge layer 12 and is linearly tapered in z with widthA(z)=A₀ -C·z. Then, along the zigzag ray path, L(z)A(z) is approximatelyequal to a constant by adiabatic invariance. If a ray starts at z=0 withL=L₀, then (A₀ -C·z)L(z) is approximately equal to L₀ A₀. The ray willleak out of the wedge layer 12 when L=cos θ_(c) where θ_(c) is thecritical angle=[1-(n₂ /n₁)² ]^(1/2). Thus, the condition for leaving thewedge layer 12 is A₀ -C·z=L₀ A₀ /cos θ_(c). This will occur at z=(A₀/C)(1L₀ /cos θ_(c)). Consequently, the density of rays emerging in z isproportional to the density of rays in the initial direction cosine L₀.For example, the density will be uniform if the initial distribution inL₀ is uniform.

In a second example, the index profile is no longer uniform but fallsoff both in x and in z. If the fall-off in z is much slower than in x,the light ray path is still almost periodic, and the above adiabaticinvariance still applies. Then, as the light ray 24 propagates in z, thepath in x,nL space is almost periodic. Therefore the maximum value ofL(z) increases and at some z may reach the critical value for escape.The z value for escape depends on the details of the index (n) profile.When this is specified, the analysis proceeds as in example one above.Thus, for a parabolic index profile, the index profile has the form n₂(x)=n₂ ₀ [1-2Δ(x/ρ)² ] for -ρ<xρ,=n₁ ² =n² ₀ [1-2Δ]for |x|>ρ. Then, thecritical angle at x=0 is still given by sin² θ_(c) =2Δ=1-(n₁ /n₀)².Then, if we have n₀ a slowly decreasing function of z, the slope θ atx=0 will slowly increase by the adiabatic invariance of φnLdx, whileθ_(c) decreases so that light rays will escape. The details of the lightray distributions will depend on how the index (n) varies with z.

Nonwedge Tapered Geometries

In the most general case the light can be input into any shape layer(e.g., parallelepiped, cylinder or non-uniform wedge), and theprinciples described herein apply in the same manner, In addition, theindex of refraction can be varied as desired in (x,y,z) to achieve theappropriate end result when coupled to means to output light to ambient.

For example, consider a disc-shaped light guide 46 which is tapered inthe radial direction r shown in FIG. 5. The direction cosines incylindrical polar coordinates are k_(r), k.sub.θ, k_(z). Light 48propagating in this guide 46 satisfies the relationship:

    φnk.sub.z dz˜constant. (adiabatic invariance)    (4)

    nrk.sub.θ =constant. (angular momentum conservation) (5)

The adiabatic invariance condition is identical with that for the wedgedevice 10, and the previous discussions pertinent to the wedge device 10also thus apply to the light guide 46. The angular momentum conservationcondition requires that as the light streams outward from source 47 withincreasing radius, the k.sub.θ value decreases. Therefore, the lightbecomes collimated in the increasing radial direction. This makes theproperties fundamentally like the wedge device 10, and the light 48 canbe made to emerge as light 52 at a selected angle to face 51, collimatedalong the z direction.

For purposes of illustration we take the guide material to have aconstant index of refraction n. For such geometries the light rays 48along the two-dimensional cross sectional plane taken along 5B--5Bbehave just as in the case of the wedge device 10 counterpart describedhereinbefore. Similarly, various additional layers 54 and 56 and othermeans can be used to achieve the desired light handling features. Forexample, for the disc light guide 46 a preferred facet array 56 is aseries of circles, concentric with the disk 46. Thus, if the facets 56are linear in cross section, the light rays 52 will emerge in adirection collimated within a full angle of 2φ times a function of theindices of refraction as in the device 10 described hereinbefore.

Tapered Luminaires with Two Low-index Layers

In another form of the invention shown in FIG. 6A, the device 10includes a first layer 61 having an optical index of refraction n₁ and afirst or top layer surface 62 and a second or bottom layer surface 64converging to establish at least one angle of inclination φ. The firstlayer 61 also includes a back surface 65 spanning the top layer surface62 and the bottom layer surface 64.

Adjacent the first layer 61 is layer means, such as a bottom transparentlayer means, like a first intermediate layer 66 of index n₂ disposedadjacent to, or underlying, the bottom layer surface 64. In addition,the layer means can embody a top transparent layer means, secondintermediate layer 81 of index n₂ disposed adjacent to the top layersurface 62. At least one of the layers 66 and 81 can be an air gap, orother gas or a transparent dielectric gap.

An air gap can be established by conventional means, such as by externalsupports, such as suspending the layers under tension (not shown) or bypositioning spacers 68 between the first layer 61 and the adjacent lightredirecting layer 70. Likewise, the spacers 68 can be positioned betweenthe first layer 61 and the second light redirecting layer 82.Alternatively, solid materials can be used for the transparentdielectric to constitute layers 66 and 81 and can improve structuralintegrity, robustness and ease of assembly. Such solid materials caninclude, for example, sol-gels (n₂ =1.05-1.35), fluoropolymers (n₂=1.28-1.43), fluoride salts (n₂ =1.38-1.43), or silicone-based polymersand adhesives (n₂ =1.40-1.45). Such solid materials for the transparentdielectric need no separate means to support or maintain it, but canresult in lower N.A. acceptance since the index is higher than for anair gap.

The layers 66 and 81 allow transmission of light received from the firstlayer 61. In this embodiment, part of the light will achieve θ_(c) firstrelative to the top layer surface 62, and light will enter the layer 81for further processing by the light redirecting layer 82. The remaininglight will thereby achieve θ_(c) first relative to the bottom layersurface 64, thus entering the layer 66 for further processing by thelight redirecting layer 70.

In one preferred form of the invention (see FIG. 6A) both the layers 66and 81 are present and can have similar, but significantly differentindices n_(2a) and n_(2b), respectively. The indices are consideredsimilar when they establish critical angles at the interfaces 62 and 64which are similar in magnitude to the wedge angle φ, for example:

    |arcsin(n.sub.2a /n.sub.1)-arcsin(n.sub.2b /n.sub.1)|<6φ                                (6)

In this case significant, but unequal, fractions of light will entereach of the layers 66 and 81 for further processing by redirectinglayers 70 and 82, respectively. The larger fraction will enter the layerhaving the higher of the two indices n_(2a) and n_(2b). The redirectinglayer 70 processes only the fraction which enters the layer 66.Therefore, the influence of the redirecting layer 70 on the outputangular distribution of light can be changed by varying the relationshipbetween the indices n_(2a) and n_(2b).

In another preferred form of the invention the layers 66 and 81 can bethe same transparent material of index n₂ <n₁. In general, lower valuesof n₂ will enhance the efficiency of the device 10 by increasing thenumerical aperture at the light input surface 65. Therefore, collectionefficiency can be maximized when the layers 66 and 81 are gaps filledwith air or other gases (with n₂ =1-1.01).

The thickness of the layers 66 and 81 can be selectively varied tocontrol the output power spatial distribution of the device 10 or toenhance its visual uniformity. For example, increasing the thickness ofthe layer 81 by 0.002"-0.030" sharply reduces non-uniformities whichtend to appear at the thicker end of the device 10. The thickness oflayers 66 and 81 can also be smoothly varied with position to influencea desired spatial distribution of the light being output (see FIG. 12L).

In one preferred form of the invention shown in FIG. 6A, the lightredirecting layer 70 includes a reflective layer 71 which reflects thelight back through the layer 66 and the first layer 61. The light isthen output into the first layer 61 through the top layer surface 62,and ultimately through the light redirecting layer 82 for furtherprocessing. The reflective layer 71 can, for example, be any combinationof a planar specular reflector, a partially or completely diffusereflector, or a faceted reflector.

Use of a planar specular reflector leads to the narrowest angulardistribution within the layer 81. Therefore, the reflector can simplifydesign of the light redirecting layer 82 when the desired output angulardistribution is unimodal. Diffuse or faceted reflectors can also be usedfor the layer 71 in order to achieve a large range of angulardistributions (see FIGS. 4H and I) or to increase uniformity (see FIG.4N). Diffuse reflectors are preferred if the desired angulardistribution has large "tails" (see, in particular, FIG. 4I). Facetedreflectors can produce a bimodal angular distribution within the layer81 (see FIG. 4H). Therefore, such faceted reflectors are preferred ifthe desired output angular distribution is bimodal. For example, abimodal "batwing" distribution is preferred from luminaires for roomillumination because it reduces glare.

In general each facet of the layer 71 can be shaped to control theangular distribution of the light reflected back through the layer 66and the first layer 61 for further processing by the redirecting layer82. The angular distribution within the device 10 will in turn influencethe angular distribution of the light output into ambient from theredirecting layer 82. For example, curved facets can be used to smoothlybroaden the angular distribution, as well as providing a diffusingeffect to improve uniformity. The reflective layer 71 can also influencethe output power spatial distribution as well as the angulardistribution. The reflectivity, specularity, or geometry of thereflective layer 71 can be varied with position to achieve a desiredoutput distribution. For example, as described hereinbefore, smallvariations in the slope (see FIG. 12L) of each element of the reflectivelayer 71 as a function of position significantly change the light outputdistribution.

The light redirecting layer 82 has an index n₃ >n₂, and is substantiallytransparent or translucent. The light in the low-index layer 81 entersthe layer 82 and is redirected into ambient. The transmissiveredirecting layer 82 also redirects the light which has been processedby reflection from the redirecting layer 71 then transmitted backthrough the low-index layer 66 and the first layer 61. The transparencyor geometry of the layer 82 can be varied with position to furtherinfluence the output spatial distribution of the device 10. In onepreferred form of the invention the redirecting layer 82 includes afaceted surface at the interface with the low-index layer 81, as shownin FIG. 6A. Light entering the layer 82 is refracted by one side 84 ofeach facet 85 as it enters, and then is totally internally reflected bysecond side 86 of each of the facets 85. In one form of the inventionthe redirecting layer 82 can be a "Transparent Right-Angle Film"(hereinafter, TRAF), which is a trademark of 3M Corp., and this productis commercially available from 3M Corp. This TRAF operates by refractionand total internal reflection to turn incident light throughapproximately a ninety degree angle, as would be desired in a typicalLCD backlighting application. The acceptance angle of the prior art TRAFis about twenty-one degrees, which is large enough to redirect a largefraction of light 75 which enters the low-index layer 81. In a morepreferred form of the invention, the facet angles are chosen to redirectmore of the light 75 which enters the low-index layer 81 by thedescribed mechanism of refraction plus total internal reflection. Eitherone or both of the facet surfaces 84 and 86 can be shaped to control theoutput angular distribution. For example, the use of curved facetssmoothly broadens the distribution, as well as providing a lightdiffusing effect which can improve uniformity.

In another preferred embodiment, the facet angle surfaces of theredirecting layer 82 can be varied progressively to compensate for thevariation in viewing angle with position, when viewed from typicalviewing distances. The details of such a compensation effect weredescribed earlier in reference to the design of the reflecting facetlayer in the embodiment shown in FIG. 2M. Similar principles can beapplied to the design of any faceted redirecting layer, includingrefracting layers and refracting/internally-reflecting layers. Examplesof embodiments which can, for example, make use of such progressivelyvaried faceted layers are shown in FIGS. 12E (layer 140), 12G (layer152), 12H (layer 166), 12K (layer 186), 12N (layer 210), 12O (layer228), and 12P (layer 246).

In another form of invention the layers 66 and 81 can have similar butslightly different indices n₂ and n₂ ', respectively. The operatingprinciples of the device 10 will be substantially similar as long as thecritical angles associated with interfaces between the first layer 61and the two layers 66 and 81 do not differ by more than the first layerconvergence angle:

    |arcsin(n.sub.2' /n.sub.1)-arcsin(n.sub.2 /n.sub.1)|<φ(7)

Therefore, in this case approximately equal fractions of the light willenter layers 66 and 81, for further processing by the redirecting layers70 and 82, respectively.

All forms of the invention can further include an output diffuser layer40, shown in phantom in FIG. 2C or transmissive or translucent diffuserlayer 83 shown in FIG. 6A. In general this diffuser layer 40 can be asurface diffuser, a volume diffuser, or at least one array of microlenses having at least a section of a cylinder (known as a "lenticulararray"). These layers 40 and 83 can increase light uniformity or broadenthe angular distribution into ambient. Lenticular arrays areadvantageous because they have low back-scattering in comparison tosurface or volume diffusers, and because they have sharper output anglecut-offs when illuminated by collimated light. Lenticular arrays alsopreferentially diffuse only those features which would otherwise run inthe general direction of the axis of each cylindrical micro lens.

In one preferred embodiment shown in FIG. 10, the light redirectinglayer 10 makes use of flat facets 111 such that the output light ishighly collimated. The desired output angular distribution is furthercontrolled by including a lenticular diffuser 112 having an appropriatefocal ratio, with its cylindrical micro lenses running approximatelyparallel to the y-axis. The lenticular diffuser 112 also diffusesnon-uniformities which would otherwise appear to be running in thegeneral direction of the y-axis. In this embodiment a second lenticulardiffuser 113 can be included to diffuse non-uniformities which wouldotherwise appear running in the general direction of the z-axis. Thissecond lenticular diffuser's micro lenses run approximately parallel tothe z-axis (see FIG. 12H and 12N). Note that the order of positioning ofthe diffusers 112 and 113 can be interchanged without loss of opticaladvantage. Similarly, the lenticular diffuser 112 and 113 can beinverted and can have concave contours rather than convex contours shownin FIG. 10. While such changes can affect the details of theperformance, the diffuser layers 112 and 113 can still provide thegeneral advantages described.

In another preferred embodiment shown in FIG. 11, the functions of theflat-faceted light redirecting layer 110 and the parallel lenticulardiffuser 112 in FIG. 10 can both be performed by a light redirectinglayer 114 having curved facets (see also, for example, FIGS. 2H, 2M and3A illustrating curved facets). These curved-facet layers redirect thelight, control the angular output by having an appropriate facetcurvature, and act as a diffuser for non-uniformities running in thegeneral direction of the y-axis. By combining these functions in asingle-layer the number of components is reduced, which improvesthickness, cost, and manufacturability. In this embodiment, a singlelenticular diffuser 115 can be included to diffuse the remainingnon-uniformities which would otherwise appear running in the generaldirection of the z-axis. This type of lenticular diffuser micro lensruns approximately parallel to the z-axis. Note that the lenticulardiffuser 115 can be inverted and can have concave contours rather thanthe convex contours shown in FIG. 10. Again, such changes can affectperformance details, but the layers 114 and 115 perform as intended.

In all embodiments using multiple micro-structured layers, the facet ornon-rational spacings of these layers described hereinbefore can bechosen to have nonrational ratios, in order to avoid undesirable Moirepatterns. Similarly, each layer's feature spacing can be designed tohave non-rational ratios with the apparatus to be illuminated, such as aliquid crystal display or charge-coupled detector (CCD) array. Each ofthe lenticular diffuser layers 113, 112 and 115 can be tilted up toabout 20° from the configuration shown in the figures in order to reduceMoire interaction between layers or with a liquid crystal display.

Similar lenticular diffusers can be used with non-wedge geometrieshaving wedge shaped cross-sections, with similar advantages if thediffuser cross sections are approximately as shown in FIGS. 10 and 11.One example is the tapered disk illustrated in FIG. 5. In this case thelenticular diffuser analogous to layer 112 in FIG. 10 would have microlenses whose axes run in concentric circles about the disk's axis ofrotations. A diffuser analogous to the layer 113 in FIG. 10 and 115 inFIG. 11 would have micro lenses whose axes emanate radially from thedisk's central axis.

Light Sources and Couplers

In a more preferred form of the invention shown in FIGS. 2A and B, afaceted layer 30 has been included for optically redirecting the light.The facets 34 can be integral to the layer 30 or a separate facet layer.Details of operation of such a faceted layer have been discussedhereinbefore. As shown further in FIG. 6A an input faceted layer 74 canalso be disposed between a light source 76 and the first layer 61. Thefaceted layer 74 can be a prismatic facet array which provides acollimating effect for input light 78 which provides brighter or moreuniform output light 80 into ambient.

Linear prisms parallel to the y-axis can improve uniformity by adjustingthe input angular distribution to match more closely the input numericalaperture. Linear prisms parallel to the x-axis can limit the outputtransverse angular distribution, and also improve output brightness whenused with a fluorescent lamp light source. In other forms of theinvention, diffusion of input light is desirable wherein a diffuser 79is used to diffuse the light distribution to spread out the light toimprove light uniformity. The diffuser 79 is preferably a lenticulararray, with cylindrical lenslets parallel to the y-axis. The diffuser 79can also be a standard surface or volume diffuser, and can be a discretefilm or coupled integrally to the wedge layer 61. Multiple prismatic ordiffuser films can be used in combination. Such a film form of thediffuser 79 and the faceted film 74 can be interchanged in position tovary their effects.

In another preferred form of the invention, a portion of a dielectrictotal internally reflecting CPC portion 100 (compound parabolicconcentrator) can be interposed between the light source 76 and thefirst layer 61 (see FIGS. 2L, 12O and 12P). The CPC portion 100 adjuststhe input light to match more closely the input numerical aperture. TheCPC portion 100 is preferably formed integrally with the first layer 61.

Reflector elements 92 and 94 shown in FIGS. 7 and 8, respectively, canbe shaped and positioned to maximize the throughput of light from thelight source 76 to the light-pipe aperture. This is equivalent tominimizing the reflection of light back to the light source 76, whichpartially absorbs any returned light. The light source 76 is typicallycylindrical and is surrounded by a transparent glass envelope 93, eachhaving circular cross-sections as shown in FIGS. 7 and 8. Typicalexamples of such light sources include fluorescent robes andlong-filament incandescent lamps. The outer diameter of the light source76 can be less than or equal to the inner diameter of the glass envelope93. FIG. 7 shows a prior art U-shaped reflector 92 formed by wrapping aspecular reflectorized polymer film around the light source 76 andmaking contact with the wedge layer 12 at each end of the film. Thereflector element 92 typically is formed into a shape which isapproximately an arc of a circle on the side of the light source 76opposite the wedge layer 12, with approximately straight sectionsconnecting each end-point of the arcwith the wedge layer 12. This mannerof coupling the reflector element 92 to the wedge layer 12 is mosteasily accomplished when the reflector element cross-section lacks sharpcorners. In general the light source 76 is not permitted to touch eitherthe wedge layer 12 or the reflectorized film, in order to minimizethermal and electrical coupling which can reduce lamp efficiency.

In one form of the present invention shown in FIG. 8, the reflectorelement 94 is advantageously designed and the light source 76 isadvantageously placed to minimize the fraction of light returned to thelight source 76, and thereby increases efficiency. In one preferredembodiment, at least a section of the reflector element 94 is shapedsuch that a line drawn normal to the surface of the reflector element 94at each point is tangent to the circular cross-section of the lightsource 76. The resulting reflector shape is known as an involute of thelight source 76.

While an involute provides maximum efficiency, other shapes cangenerally be more easily manufactured. Polymer films can be readily bentinto smooth curves which include almost semicircular arcs, as describedabove. It can be shown that when the cross-section of the light source76 and semicircular section of the reflector element 92 are concentricas shown in FIG. 7, then the semicircular section of the reflectorelement 92 will return all incident rays to the light source 76, leadingto poor efficiency. Such inefficiency is a general property ofself-absorbing circular sources and concentric semicircular reflectors.This general property can be derived from simple ray-tracing or theprincipal of skew invariance. Even if the reflector element 92 is notperfectly circular, each portion of the reflector element 92 will tendto return light to the light source 76 if the cross-section of the lightsource 76 is centered near the center of curvature of that reflectorsection.

In another preferred embodiment, the cross-section of the reflectorelement 94 in FIG. 8 includes one or more almost semicircular arcs, andefficiency is increased by displacing the center of the light source 76away from the center of curvature of the reflector element 94.Ray-tracing and experiments have shown that such preferred embodimentscan be determined using the following design rules:

1. The cross-section of the reflector element 94 has a maximum extent inthe x-dimension equal to the maximum thickness of the wedge layer 12 (orlight pipe);

2. The cross-section of the reflector element 94 has no optically sharpcorners;

3. The radius of curvature of the reflector element 94 is as large aspossible; and

4. The light source 76 is as far as possible from the wedge layer 12,but is far enough from the reflector element 94 to avoid contact withworst-case manufacturing variations.

FIG. 8 shows an example of a coupler which satisfies these abovedescribed design rules for the light source 76 with inner diameter=2 mm,outer diameter=3 mm, thickness of the wedge layer 12 (or light pipe)=5mm, and manufacturing tolerances which permit a 0.25 mm spacing betweenthe reflector element 94 and the outer diameter of the glass envelope93. In this example of a preferred embodiment the radius of curvature ofthe reflector element 94 is 2.5 mm, and the center of the light source76 is displaced by 0.75 mm away from the aperture of the wedge layer 12.A coupler constructed according to this design was found to be 10-15%brighter than the comparable concentric coupler shown in FIG. 7.

The involute and the U-shaped reflector elements 92 and 94 previouslydescribed are designed to output light to the aperture of the wedgelayer 12 with angles approaching ±90 degrees relative to the aperturesurface normal. In another preferred embodiment, the reflector element94 is shaped to output light with an angular distribution which iscloser to the N.A. of the device 10. As shown in FIGS. 6B and 6C, suchshapes as the reflector element 94 can include other geometries, suchas, a compound parabolic source reflector 86 and a nonimagingillumination source reflector 88. An example of the source reflector 88is described in copending Ser. No. 07/732,982 assigned to the assigneeof record of the instant application, and this application isincorporated by reference herein.

In another embodiment of the invention shown in FIGS. 6D, 12L, 12N, and12O, the wedge layer 90 has a non-monotonic varying wedge crosssectional thickness over various selected portions of the wedge shapedcross section. It has been determined that one can exert control overthe light distribution being output by control of this cross section.Further, it has been determined that optical boundary effects, as wellas intrinsic light source effects, can combine to give an output lightdistribution with unwanted anomalies. One can therefore also compensatefor these anomalies, by providing a wedge cross section with nonlinearchanges in the actual dimensions of the wedge layer 90, for example,near the thicker end which typically receives the input light. Bycontrol of these dimensions one can thus have another degree of freedomto exert control over the light distribution, as well as providevirtually any design to compensate for any boundary effect or lightsource artifact. Furthermore, one can vary the index of refractionwithin the wedge layer 90 in the manner described hereinbefore to modifythe distribution of light and also compensate for light input anomaliesto provide a desired light distribution output.

Manufacture of Luminaire Devices

In one form of the invention, manufacture of the device 10 can beaccomplished by careful use of selected adhesives and laminationprocedures. For example, the wedge layer 12 having index n₁ can beadhesively bonded to the first layer 28 having index n₂. An adhesivelayer 60 (see FIG. 3B) can be applied in liquid form to the top surfaceof the first layer 28, and the layer 28 is adhesively coupled to thebottom surface 16 of the wedge layer 12. In general, the order ofcoupling the various layers can be in any given order.

In applying the layer 12 to the layer 28 and other such layers, theprocess of manufacture preferably accommodates the formation of internallayer interfaces which are substantially smooth interfacial surfaces. Ifnot properly prepared such internal layers can detrimentally affectperformance because each interface between layers of different indicescan act as a reflecting surface with its own characteristic criticalangle. If the interfacial surfaces are substantially smooth, then thedetrimental effect of uneven surfaces is negligible. Therefore ineffectuating the lamination of the various layers of the device 10, themethodology should utilize adhesives and/or joining techniques whichprovide the above described smooth interfacial layers. Examples oflamination processes include, without limitation, joining withoutadditional adhesive layers, coatings applied to one layer and thenjoined to a second layer with an adhesive and applying a film layer withtwo adhesive layers (one on each layer surface to be joined to theother).

In a preferred embodiment lamination of layers is done without anyadditional internal layer whose potential interfacial roughness willdistort the light distribution. An example of such a geometry for thedevice 10 can be a liquid layer between the wedge layer 12 and thesecond layer 30. This method works best if the first layer 29 (such asthe liquid layer) acts as an adhesive. One can choose to cure theadhesive either before, partially or completely, or after joiningtogether the various layers of the device 10. The optical interface isthus defined by the bottom surface of the wedge layer 12 and the topsurface of the second layer 30.

In another embodiment wherein a coating is used with an adhesive layer,the first layer 28 can be the coating applied to the second layer 30.Then, the coated film can be laminated to the wedge layer 12 in a secondstep by applying an adhesive between the coated film and the wedge layer12. It is preferable to apply the low index coating to the second layer30 rather than directly to the wedge layer 12 since the second layer 30is typically supplied in the form of continuous film rolls. In practiceit is more cost effective to coat such continuous rolls than to coatdiscrete pieces. With this methodology it is more convenient to controlthickness of the applied low index layer.

In another embodiment, the second layer 30 is manufactured in such a waythat it adheres to the first layer 28 directly without use of additionaladhesives. For example, the second layer 30 can be manufactured byapplying a layer of polymer material to the first layer 28, and thencasting this material to have the desired second layer geometry. Inanother example, the first layer 28 can serve as a carrier film duringthe embossing of the second layer 30. By use of appropriate temperaturesduring the embossing process, the second layer 30 can be heat-fused tothe first layer 28. Such heat-fusing can be accomplished using aconventional FEP first-layer film by embossing at almost five hundreddegrees F or higher.

In a further embodiment using a film and two adhesives, the first layer28 can be an extruded or cast film which is then laminated to the wedgelayer 12, or between the wedge layer 12 and the second layer 30 usingadhesive between the two types of interfaces. In order to minimize thedetrimental light scattering described hereinbefore, the adhesive layershould be flat and smooth. The film can be obtained as a low indexmaterial in commercially available, inexpensive forms. Such additionaladhesive layers can increase the strength by virtue of the multi-layerconstruction having adhesive between each of the layers.

In the use of adhesive generally, the performance of the device 10 isoptimized when the index of the adhesive between the wedge layer and thefirst layer is as close as possible to the index of the first layer 28.When the critical angle at the wedge/adhesive interface is as low aspossible, then the light undergoes a minimal number of reflections offthe lower quality film interface before exiting the device 10. Inaddition, the index change at the surface of the first layer film isminimized which decreases the effects of film surface roughness.

Manufacture of faceted surfaces can be accomplished by micro-machining amold using a master tool. Machining can be carried out by ruling with anappropriately shaped diamond tool. The master tool can be replicated byknown techniques, such as electroforming or casting. Each replicationstep inverts the shape of the desired surface. The resulting mold orreplicates thereof can then be used to emboss the desired shape in thesecond layer 30. A directly ruled surface can also be used, but theabove described embossing method is preferred. Known "milling" processescan include chemical etching techniques, ion beam etching and laser beammilling.

In yet another method of mechanical manufacture, the faceted surface 34(see FIGS. 2B and 2M, for example) is manufactured by a welding process,such as embossing or casting, using a hard tool which has on one surfacethe inverse of the profile of the desired faceted surface 34. Therefore,the manufacturing problem reduces to the matter of machining anappropriate tool. Usually the machined tool is used as a template toform the tools actually used in the casting or embossing process. Toolsare typically replicated by electroforming. Since electroforming invertsthe surface profile, and electroforms may be made from otherelectroforms, any number of such inversions can be accomplished and thedirectly machined "master" can have the shape of the faceted surfaces 3Aor its inverse.

The tooling for the faceted surface 34 can be manufactured bysingle-point diamond machining, wherein the distance between cuttingtool and the work is varied to trace out the desired profile. Thediamond cutting tool must be very sharp, but in principle nearlyarbitrary profiles can be created. A given design can also requirespecific adaptations to accommodate the non-zero radius of the cuttingtool. If curved facet surfaces are required, then circular arcs arepreferred to facilitate fabrication. The cutting tool is moved throughthe cutting substrate and cuts a groove having the approximate shape ofthe tool. It is desirable to machine the entire piece using a singlediamond tool. When this method is used for making a "focusing" type ofthe faceted surface 34, the variable groove profile therefore should bedesigned such that the various groove profiles can be machined by thesame tool. The required shape variations can still be accomplished byvarying the angle of the tool, as well as the groove spacing and depth.

Design of the faceted surface 34 preferably satisfies a few generalconstraints:

1. Approximately linear variation in the center of the illuminationangular distribution as a function of position. A variation of 11degrees (±5.5°) from top to bottom of typical computer screens iseffective;

2. The width of the variable angular distribution of light output shouldbe approximately proportional to the local illuminance in order toachieve approximately uniform brightness to an observer. Examples givenbelow show the spatial distribution is approximately uniform, so theangular cones have approximately uniform width; and

3. Spacing between grooves of the facets 38 should be large enough orirregular enough to avoid diffraction effects, but also be chosen toavoid Moire patterns when used with an LCD panel. In practice theserequirements limit the allowed spatial variations.

In the manufacture of the device 10, for example, the viewing angledepends on the tilt and curvature of each of the facets 38. Focusing isaccomplished by rotating the facet structure as a function of position.Using the example of a 150 mm screen viewed from 500 mm away, theillumination cone can be varied by 17 degrees (i.e., ±8.5 degrees) fromtop to bottom. For typical materials, acrylic and FEP, this requires thefacet structure to rotate by approximately 5.7 degrees from top tobottom of the screen 89 (see FIG. 9B).

Design constraints can result when limitations (1)-(3) are combined withthe need to machine variable curved grooves with a single tool. Forexample, maintaining a constant angular width (Constraint #1) at aconstant cutting depth requires a compensating variation in groovespacing or groove depth. Specifically, a linear change in groove spacingcan reduce the brightness variation to a negligible level when the formtool which cuts the groove is shaped so that portions of each curvedreflector facets (see FIG. 2M) are shadowed by the top edge of theadjacent facets. This spacing variation can be small enough to satisfyConstraint #3.

Further methods of manufacture can include vapor deposition, sputteringor ion beam deposition of the first layer 28 since this layer can bequite thin as described hereinbefore. Likewise, the second layer 30 canbe controllably applied to form the faceted layer 30 shown in FIG. 2B(such as by masking and layer deposition).

Wedge Light Pipe as a Simple Collimator Device

In the most general embodiment the wedge layer 12 can function in thecontext of the combination as a simple collimating optical element. Thesubstantially transparent wedge layer 12 has an optical index ofrefraction n₁ and the top surface 14 and the bottom surface 16 convergeto establish at least one angle of inclination φ (see FIG. 15). Thewedge layer 12 also includes the back surface 20 spanning the topsurface 14 and the bottom surface 16. Adjacent to the wedge layer 12 isthe transparent first layer 28 having index of refraction n₂ includingan air gap. Adjacent to the first layer 28 is a specular reflectivelayer, such as the faceted surface 34 of the second layer 30.

Substantially uncollimated light is introduced through the back surface20 by the source 22. The light propagates within the wedge layer 12,with each ray decreasing its incident angle with respect to the top andbottom surfaces 14 and 16 until the incident angle is less than thecritical angle θ_(c). Once the angle is less than θ_(c), the ray emergesinto ambient. Rays which emerge through the bottom surface 16 arereflected back into the wedge layer 12 and then output into ambient. Byvirtue of the angle-filtering effect previously described, the outputlight is collimated within a cone of angular width approximately:

    Δθ≅2φ.sup.1/2 (n.sup.2 -1).sup.1/4(8)

An area 99 to be illuminated lies beyond the end of the wedge layer 12and substantially within the above-defined cone of width Δθ.

In another preferred embodiment a light-redirecting means can bepositioned beyond the end of the wedge layer 12 and substantially withinthe above-defined cone of width Δθ. The light-redirecting means can be alens, planar specular reflector, or curved reflector. Thelight-redirecting means reflects or refracts the light to the area to beilluminated. Further details and uses of such redirecting means, such aslenticular diffusers, will be described hereinafter.

In the embodiments of FIG. 6 having two air gaps or transparentdielectric layers, the light redirecting layers are independent, andthus one can construct devices having layers of different types. Forexample, the use of two transmissive redirecting layers is preferredwhen light is to be emitted from both sides of the device 10 or whenevermaximum collimation is desired. Examples of the redirecting layer 82 ingeneral for all inventions for two redirecting layers can include theexamples in FIG. 12 where the letter in parenthesis corresponds to theappropriate figure of FIG. 12: (a) diffraction gratings 120 or ahologram 122 in FIG. 12A, (b) two refracting facet layers 124 withdiffusers 126 in FIG. 12B, (c) two faceted layers 128 with facets 130designed to refract and internally reflect light output from the wedgelayer 12; such facets 130 are capable of turning the light outputthrough a larger angle than is possible by refraction alone; (d) tworefracting single facet layers 132 (prisms); (e) a top surfaceredirecting layer for the wedge layer 12 having a refracting singlefacet layer 134 with a curved output surface 136 for focusing. A bottomsurface 138 includes a redirecting layer for refracting and internallyreflecting light using a faceted layer 140; facet angles are varied withposition to focus output light 142 at F; (f) a top surface redirectinglayer 144 comprised of a refracting faceted layer 146 and a bottomredirecting layer comprised of a refracting/internally reflecting layer148 with narrow angle output for the light, and a diffuser layer 150 canbe added to smoothly broaden the light output angular distribution; (g)a top surface redirecting layer of refracting/internally reflectingfaceted layer 152 with refracting surfaces 154 convexly curved tobroaden the output angular distribution; the facet angles can be variedwith position and thereby selectively direct the light output angularcones to create a preferred viewing region at a finite distance; thisarrangement can further include a transverse lenticular diffuser 156 todiffuse nonuniformities not removed by the curved facet layer 152; thebottom redirecting layer comprises a refracting/internally reflectingfaceted layer 158 with a reflecting surface 160 being concavely curvedto broaden the light output angular distribution in a controlled manner;(h) a top redirecting layer, including a refracting faceted layer 162with curved facets 164 to broaden the output angular distribution in acontrolled manner and to improve uniformity; a bottom redirecting layer,including a refracting/internally-reflecting faceted layer 166 with flatfacets 168 for narrow-angle output, with facet geometry varied withposition to focus output light at a finite distance; a parallellenticular diffuser 170 can be used to smoothly broaden the outputangular distribution in a controlled manner and to improve uniformity;the transparent image shown in phantom can be printed on or adhesivelybased to a lenticular diffuser; a transverse lenticular diffuser 172 isused to diffuse nonuniformities not removed by the parallel lenticulardiffuser 170. The combination of a focused flat-faceted layer 166 andthe diffuser 170 cooperate to create a preferred viewing zone at afinite distance, similar to using focused curved facets. Also shown isan LCD component 173 (in phantom) usable with this and any other form ofthe device 10 for illumination purposes.

In other architectures, one transmissive and one reflective redirectinglayer can be combined. These are combinations of reflective redirectinglayers with the various types of transmissive redirecting layersdiscussed above. Reflective redirecting layers can be specular,partially diffuse, diffuse, faceted or any combination thereof. Thesearchitectures are preferred when light emission is desired from one sideonly, or in some cases when minimum cost is paramount. Examples of sucharchitectures are in FIG. 12: (i) a bottom surface specular reflector174 combined with a top layer transmission diffraction grating ortransmission hologram 176; (j) a bottom surface specular reflector 178combined with a top surface refracting faceted layer 180, with adiffuser 182 (shown in phantom in FIG. 12J and an interveningimage-forming layer 171; (k) a bottom layer specular reflector 184 witha top layer refracting/internally-reflecting faceted layer 186, withfacet geometry being varied with position to focus output light at afinite distance; a diffuser 188 is shown in phantom; (l) a bottom layerspecular reflector 190 with a top layer refracting/internally-reflectingfaceted layer 192, and curved facets 194 are used to smoothly broadenthe angular output of light in a controlled manner and to improveuniformity. The thickness of the wedge layer 12 and of both top andbottom surface low-index layers 196 (e.g., air gaps) are varied toinfluence the light output spatial distribution; (m) a bottom reflector198 is partially specular, partially diffuse to improve uniformity; FIG.12M shows the initial reflector section made controllably diffuse byaddition of an integral lenticular diffuser 200; the diffuser 200 isdesigned to selectively reduce nonuniformities which would otherwiseappear in the output near the thicker end, and running in the generaldirection of the y-axis; also included is a top redirecting layer 202which is refracting/internally-reflecting and has a reflecting surfacewhich is curved; and (n) a bottom reflector layer 204 which is partiallyspecular, partially diffuse to improve uniformity; FIG. 12N shows theinitial reflector section 206 which is slightly roughened to reducespecularity, and thereby selectively reduces nonuniformities which wouldotherwise appear in the output near thicker end 208; a top redirectinglayer 210 is used which is refracting/internally-reflecting with aflat-faceted layer 212, and the facet geometry is varied to redirectlight from each facet to a common focus at finite distance; a transverselenticular diffuser 213 is shown in phantom; a parallel lenticulardiffuser 214 is used to smoothly broaden the output angular distributionin a controlled manner, converting the focal zone of the flat-facetedlayer 212 to a wider preferred viewing zone; the lenticular diffuser 213also improves uniformity; an LCD display 216 or other transparent imageis show in phantom; (o) in a preferred embodiment an eccentric coupler218 uses a uniformity-enhancing lenticular diffuser 220 shown in phantomin FIG. 12O. A converging tapered section 222 or CPC (integral to thewedge layer) transforms the output angular distribution to match moreclosely the input N.A. of the wedge layer 12. The wedge layer 12thickness is smoothly varied to influence output spatial distributionand improve uniformity; a bottom redirecting layer 224 is a specular orpartially diffuse reflector; a top redirecting layer 226 is arefracting/internally-reflecting faceted layer 228 with reflectingsurfaces 230 convexly curved to smoothly broaden output angle in acontrollable manner; facet geometry is varied with position toselectively direct the angular cone of light from each face to create apreferred viewing zone 232 at a finite distance; a transverse lenticulardiffuser 234 is shown in phantom; an LCD display 236 or othertransparent image is also shown in phantom; the more convergingN.A.-matching section is advantageous in combination with the facetedredirecting layers, because the redirecting and low-index layers do notneed to overly the more converging section; therefore, the inputaperture (and thus efficiency) of the device 10 is increased withminimum increase in total thickness of the device; (p) another preferredembodiment for LCD backlighting uses an eccentric coupler with auniformity-enhancing diffuser shown in phantom in FIG. 12P; a converginghalf-tapered section 240 or half-CPC (integral to the wedge layer 12)transforms a coupler output angular distribution to match more closelythe input N.A. of the wedge layer 12. A diffuser 239 (in phantom) canalso be interposed between light source 217 and the wedge layer 12. Thesufficiently truncated half-CPC 240 is just a simple tapered section. Abottom reflector 242 which is partially specular, partially diffuse isused to improve uniformity; FIG. 12P further shows an initial reflectorsection 244 which is slightly toughened to reduce specularity, oralternatively shaped into a series of parallel reflective grooves, whichthereby selectively reduces nonuniformities which would otherwise appearin the output near the thicker end; a top redirecting layer 246 is arefracting/internally-reflecting faceted layer 248, with refractingsurfaces 250 convexly curved to smoothly broaden output angle in acontrollable manner; facet geometry is varied with position toselectively direct angular cones of light from each facet to create apreferred viewing zone at a finite distance; a transverse lenticulardiffuser 252 is shown in phantom. Also included is an LCD display 254 orother transparent image shown in phantom.

The more converging N.A.-matching section (such as half tapered section240) is advantageous in combination with the faceted redirecting layers,because the redirecting and low-index layers do not need to overly themore converging section; therefore, the light-accepting aperture of thedevice 10 is increased without increasing the total thickness. Theadvantage is also conferred by the fully-tapered section 222 shown inFIG. 12O; but in comparison the half-tapered section 240 in FIG. 12Pprovides greater thickness reduction on one side, at the expense ofbeing longer in the direction of taper for equivalent N.A.-matchingeffect. It can be desirable to concentrate the thickness reduction toone side as shown, because the top surface low-index layer can be madethicker to improve uniformity. This configuration can be more easilymanufactured because the bottom reflector layer can be integral to thecoupler reflector cavity, without need to bend a reflective film arounda corner; (q) a bottom specular or diffusely reflecting layer 256 can becombined with single-facet refracting top layer 258 in yet anotherembodiment (see FIG. 12Q); and (r) in cases for interior lighting usage,a bimodal "bat-wing" angular light distribution 260 is preferred; inFIG. 12R is shown a top refracting layer 262 with facets 264 and has acurved front surface 266 to smoothly broaden angular output and improveuniformity, with output light directed primarily into a forwardquadrant; a bottom reflecting layer 268 reflects light primarily througha back surface of a top redirecting layer, with output directedsubstantially into a backwards quadrant.

As understood in the art the various elements shown in the figures canbe utilized with combinations of elements in tapered luminaire devices.Examples of two such combination geometries are shown in FIGS. 13 and14, each figure also including features specific to the geometry shown.As illustrated in FIG. 13, two wedges 276 can be combined and formedintegrally. This combination can provide higher brightness than a singlewedge having the same extent because it permits two light sources tosupply light to the same total area. While brightness is increased forthis device, efficiency is similar because two sources also requiretwice as much power as one source. A redirecting film 272 with facets274 can be a single, symmetric design which accepts light from bothdirections as shown. Alternatively, the redirecting film 272 can have adifferent design for each wing of the butterfly.

In FIG. 14 is shown a three dimensional rendition of a tapered disk 270,such as shown in FIG. 5, and is sectioned to show the appearance of thevarious layers. A faceted redirecting layer 280 comprises concentriccircular facets 282 overlying a tapered light-pipe portion 284. Directlyover a light source 288, overlying the gap at the axis of the light-pipeportion 284, the redirecting layer 280 takes the form of a lens (aFresnel lens 280 is shown, for example). Directly below the light source288 is a reflector 290 positioned to prevent light from escaping and toredirect the light into the light-pipe portion 284 or through the lens.At least one opening is provided in the reflector to permit passage ofelements, such as wires or light-pipes.

Use of Imaging or Colored Layers

All embodiments of the invention can incorporate one or more layerswhich have variable transmission to form an image, or which impart colorto at least a portion of the angular output. The image-forming layer caninclude a static image, such as a conventional transparent display, or aselectively controlled image, such as a liquid crystal display. Theimage-forming or color-imparting layer can overlay one of theredirecting layers, or alternatively it can comprise an intermediatelayer between one of the low-index layers and the associated redirectinglayer, or an internal component of a redirecting layer. For example,overlying image-forming layers 129 are shown in phantom in FIGS. 12C and12G. Examples of an internal image-forming layer 171 are shown in FIGS.12H and 12J.

In one preferred embodiment, the image-forming layer (such as 129 and170) is a polymer-dispersed liquid crystal (PDLC) layer. By properarrangement of the layers, the image or color may be projected from thedevice within selected portions of the output angular distribution. Theimage or selected color can be substantially absent in the remainingportions of the output angular distribution.

Bi-modal Reflective Wedge for LCD Panel Illumination

In some applications it is desired to illuminate a single LCD panelselectively with either ambient light or by active back-lighting. Inthese applications ambient illumination is selected in well-litenvironments in order to minimize power consumption by the display. Whenavailable environmental illumination is too low to provide adequatedisplay quality, then active backlighting is selected. This selectivebi-modal operating mode requires a back-illumination unit which canefficiently backlight the LCD in active mode, and efficiently reflectambient light in the alternative ambient mode.

The most widespread prior art bi-modal liquid crystal display is the"transflective display" 101, such as is shown in FIG. 16B. This approachuses a conventional backlight 102 and a transmissive LCD panel 103, withan intervening layer 104 which is partially reflective and partiallytransmissive. In order to achieve adequate ambient mode performance, itis typically necessary for the intervening layer 104 to be 80-90%reflective. The resulting low transmissivity makes the transflectivedisplay 101 inefficient in the active mode of operation.

Another embodiment of the invention is shown in FIG. 17. This embodimentoutperforms prior art transflective displays in the active mode, anddemonstrates comparable performance in the ambient mode. In thisembodiment the wedge layer 12 (index=n₁) having the bottom surface 16 iscoupled to a transparent layer 28 of index n₂ <n₁, which can be an airgap. The n₂ layer is coupled to a partially diffuse reflector layer 105.This reflector layer 105 is, for example, preferably similar to thereflectors used in conventional LCD panels used in ambient mode only, asshown in FIG. 16A. Overlaying the wedge layer top surface 14 is afaceted redirecting layer 106, such as a lenticular diffuser with microlenses approximately parallel to the y-axis. A liquid crystal displaypanel 107 overlays the faceted redirecting layer 106. The back surface20 of the wedge layer 12 is coupled to the light source 22.

The lenticular redirecting layer 106 and the wedge-layer 12 aresubstantially transparent to the incident and reflective light, so thatin ambient mode the device 10 operates in a manner similar toconventional ambient-mode-only displays. When an active mode isselected, the light source 22 is activated, and the multiple layers actto spread the light substantially uniformly over the device 10 by virtueof the relationship between the indices of refraction and convergenceangles of the layers, as described before. The resulting uniformillumination is emitted through the top surface 14 of the wedge layer12. In a preferred embodiment, the reflector layer 105 is nearlyspecular in order to maximize ambient-mode performance. In thispreferred embodiment the light emitted from the top surface is emittedlargely at grazing angles, unsuitable for transmission by the LCDdisplay panel 107. The redirecting layer 106 redirects a fraction ofthis light by a combination of refraction and total internal reflection,as described hereinbefore. The redirecting layer 106 is preferablydesigned such that at least 10-20% of the light is redirected intoangles less than 30 degrees from the LCD normal, because typically theLCD transmission is highest in this angular range. It is sufficient todirect only a fraction of the back-illumination into suitable angles,because the prior art transflective display is quite inefficient in theactive mode of operation.

Processing Polarized Light

In another aspect of the invention, the light being processed by theoptical device 10 has an inherent polarization (such as, linear,circular and elliptical) that can be used to advantage in improving theillumination from a liquid crystal display ("LCD") system or otheroutput which depends on using polarized light. In a system which employsan LCD, it is necessary to remove one type of polarized light 308 andpass to the LCD layer only the other type of polarized light. Forexample in FIG. 30 a conventional polarization layer 312 preferentiallyabsorbs one polarization of light amounting to about one-half the inputlight from light source 306, with the preferred polarization light beingtransmitted to LCD layer 316. The polarized light of the properpolarization is processed by the liquid crystals and a second polarizer314 in the desired manner to provide the displayed feature of interest.In such a conventional system about half the light from the light sourceis "unwanted" and thus is lost for purposes of providing an LCD outputof interest. Consequently, if a means could be found to utilize bothtypes of polarized light (not removing light of an unwantedpolarization), a substantial gain in efficiency and brightness canresult for the liquid crystal display. The subject invention is directedin part to that end, and the following embodiments are preferredstructures and methods for accomplishing that goal.

In the most general explanation of a polarization filter, referring toFIG. 30B, the function of a polarization filter layer 307 is to take theinput light 308 consisting of two polarization states of type 1 and 2and create transmitted light 309 consisting of polarization states 3 and4 and reflected light 311 consisting of polarization states 5 and 6.This can be related to our specific references hereinafter to a "first"and "second" state as "states" 1,3 and 5 as the "first polarizationlight 218" and 2,4 and 6 as the "second polarization" light 220. Thus,we assume that the form of states 3 and 5 are chosen so that they alonespecify the light that is transmitted and reflected due to the lightportion incident in polarization state 1, and let states 4 and 6 beassociated with polarization state 2. However, the form of thepolarization states need not be related in any more specific way. Forsome range of incident angles over some spectral wavelength range andfor some specific selection of input polarization states, thepolarization filter layer 307 processes the input light 308 and producesoutput light 309 with a specific total power relationship. If we definethe powers (P_(i)) in each of the polarization states (i, wherei=1,2,3,4,5,6), the condition is: ##EQU3##

By definition, any layer which exhibits the above characteristics over asuitable angular and spectral range is a form of the polarization filterlayer 307. Generally, the polarization states considered can be ofarbitrary type such as linear, circular, or elliptical. In latersections we will quantify the performance of the polarization filterlayer 307 by a degree of polarization (P_(T)) defined as: ##EQU4##

For lossless layers, the transmittance is related to the reflectance, R,by

    T.sub.31 =1-R.sub.51,T.sub.42 =1-R.sub.62

where

    R.sub.51 =R.sub.5 /R.sub.1 and R.sub.62 =R.sub.6 /R.sub.2

There are a variety of implementations of a layer medium which has theproperties described above for the polarization filter layer 307. Theseinclude, but are not restricted to, implementations containing one ormore of the following types of layers: (1) thin-film layers produced bycoating, extrusion, or some other process which are eithernon-birefringent or birefringent and are designed to operate as opticalinterference coatings; (2) "thick" film layers which are more than asingle quarter wavelength optically thick somewhere in the spectral bandof interest and may be produced by stacking, coating, extrusion,lamination, or some other process and are designed to operate as aBrewster Stack even when the angles and indexes do not exactly match theBrewster angle conditions; (3) a combination of the thin-film and thickfilm approaches; (4) correlated, partially correlated, or uncorrelatedsurface roughness or profile which results in polarization dependentscattering and produced by any method including etching, embossing,micro-machining, or other method; (5) and layers based on dichroicmaterial. In general, an aggregate layer formed by one or more the abovelayer types is a suitable form of the polarization filter layer 307layer if it satisfies the general functional specifications describedabove for polarization filter layers.

The implementations of the polarization filter layer 307 can consist ofeither thin-film or thick-film birefringent or non-birefringent layers.Particular examples and discussion of birefringent layers will beprovided in a labeled subsection presented hereinafter.

One example embodiment of a thick film form of the polarization filterlayer 307 is based on a specific design center wavelength (λ₀) and aspecific design operating angle (θ_(inc)) as shown in FIG. 30C and basedon isotropic planar layers. Layers 313 in this design example consist oftwo types of alternating layers, called high (H) layer 314 and low (L)layer 315 of optical refractive index n_(H) and n_(L) respectively. FromSnell's law, we know the angle with respect to the surface normals(θ_(L), θ_(H)) at which the light 317 are traveling in any of the layer313 in terms of the refractive indexes of the layers (n_(inc), n_(L),n_(H)) if we know the incidence angle. This implies:

    n.sub.inc sin θ.sub.inc =n.sub.L sin θ.sub.L

    n.sub.inc sin θ.sub.inc =n.sub.H sin θ.sub.H

For p-polarized form of the light 317 incident on an interface betweentwo optically isotropic regions, there is an angle called the Brewster'sAngle at which the reflectivity of the interface is zero. This anglemeasured to the surface normal (θ_(H/L), θ_(L/H)) is: ##EQU5##

The reflectivity of the interfaces to s-polarized light at Brewster'sAngle can be significant. The layers 313 which preferentially transmitsthe p-polarization state is designed by spacing these interfaces byquarter-wave optical thicknesses. Such quarter wavelength thicknesses(t_(L), t_(H)) are given by: ##EQU6##

One can show that the H and L indexes of refraction are related by thedesign equation: ##EQU7##

As an example, consider the specific case of:

    n.sub.H =1.5,n.sub.inc =1.0,θ.sub.inc =80°,λ.sub.0 =500 nm.

This implies that the design index of refraction of the low index layerand the physical thicknesses of the low and high index layers 314 and315 should be respectively n_(L) =1.31,t_(L) =145 nm, t_(H) =110 nm.These can be achieved by using sputtered glass and vacuum depositedlithium chloride for n_(H) =1.5 and n_(L) =1.31, respectively. Assumingthat the design is a matched design as in FIG. 30C, with the layers 313surrounded by an index of refraction of 1.5, the reflectivity can beeasily calculated with the well-known Rouard's Method. This matchingassumption is quite general as the outer surfaces could always beanti-reflection coated. The reflectivity for a variety of basic layercounts for the layers 313 is shown in Table 1 below:

                  TABLE 1                                                         ______________________________________                                        Performance data for the polarization filter layer 307                        Layer Count     s-Reflectivity                                                                           P.sub.T                                            ______________________________________                                         1              0.069      0.036                                               5              0.45       0.29                                               11              0.85       0.75                                               15              0.95       0.90                                               21              0.99       0.98                                               ______________________________________                                    

There are a variety of similar alternative designs. More than a singlerefractive index may be used as part of the thin-film structure of thelayers 313. The surrounding layers need not be air and the exact numberof low and high index layers is variable. The carrier or substrate couldhave other refractive index values. The layers 313 can be varied fromtheir quarter-wave thickness at the design angle and the wavelength soas to improve spectral and angular bandwidths. In fact, the operabilityof the layers 313 can be quite broad band and the Brewster angle designdoes not have to be followed with great precision in index and angle.For example, you can trade off s-reflectivity with p-transmission bychanging refractive indexes. The whole system can be flipped withoutchanging its function.

A variety of preferred embodiments include at least two layers ofdifferent indices. Such arrangements have the n_(H) and n_(L) such thatn_(H) /n_(L) >1.15 in order to minimize the number of layers requiredfor high polarization selectivity. Further, optical interference is mostpreferably used to enhance performance by using at least one layer withindex n and thickness t such that 50 nm/(n² -1)^(1/2) <t<350 nm/(n²-1)^(1/2). This relationship derives from the equations providedhereinbefore regarding t_(L) and t_(H), by noting that the wavelength isin the visible light range 400 nm to 700 nm, that the incident light isnear the critical angle so that n sin θ≈1 and optical interferenceeffects are promoted by layers with an optical thickness between 1/8 and1/2 of the light wavelength. Materials and methods for fabricating suchlayers are well-known in the art of multi-layer dielectric coatings.

The Brewster Stack approach is similar to the thin-film approachdescribed above except that the layers are many wavelengths thick andtend to function largely on the basis of the incoherent addition of thewaves rather than the coherent effect that occurs in opticalinterference coatings. The design of this form of the polarizationfilter layer 307 is the same as the design of the thin-film polarizerdescribed above except that layer thicknesses are not important as longas they are at least several wavelengths thick optically. The lack ofoptical thickness effects suggests that the performance of the BrewsterStack implementation should generally be less sensitive to spectralwavelength and angular variations. The transmission ratio defined interms of the transmission of the s and p polarized light (T_(s),T_(p))of the set of N layer pairs in the geometry of FIG. 30D can be estimatedusing the approximate formula: ##EQU8##

The results of applying this formula to a geometry with varying numbersof layer pairs is shown in Table 2 below:

                  TABLE 2                                                         ______________________________________                                        Performance data for a Brewster Stack                                         Form of the Filter Layer 307                                                  Layer Pairs      T.sub.s /T.sub.p                                                                      P.sub.T                                              ______________________________________                                         1               0.9755  --                                                   20               0.61    --                                                   50               0.29    0.55                                                 100              0.08    0.85                                                 ______________________________________                                    

Generally speaking, this type of the polarization filter layer 307requires much larger index differences and many more layers for the samereflectivities. There is no sharp dividing line between the thin-filmdesign and the Brewster stack approach. As thickness increases,coherence effects slowly decrease and beyond some point which isdependent on the spectral bandwidth of the light signal, the coherenceeffects become small compared to incoherent effects. These examplesdescribed herein are simply the extreme of cases of the coherent andincoherent situations.

In FIG. 19 are shown variations on one form of a polarized lightluminaire system 204. In particular, in FIG. 19B, the system 204includes a base layer 206 having a wedge-shaped, cross-sectional areawith optical index of refraction n₁, and a first surface 208 and secondsurface 210 converging to define at least one angle of inclination φ.The base layer 206 further includes a back surface 211 spanning thefirst surface 208 and the second surface 210. Light 212 injected by asource (not shown) through the back surface 211 reflects from the firstand second surfaces and exits the base layer 206 when the light 212decreases its angle of incidence relative to a normal to the first andsecond surfaces with each reflection from the surfaces 208 and 210 untilthe angle is less than a critical angle θ_(c) characteristic of aninterface between the base layer 206 and a first layer means, such as alayer 214. This layer 214 includes at least a layer portion having indexn₂ less than n₁ disposed beyond the second surface 210 relative to thebase layer 206. The first layer 214 enables the light 212 to enter thefirst layer 214 after output from the base layer 206 when the light 212in the base layer 206 achieves the angle of incidence less than thecritical angle θ_(c) characteristic of an interface between the baselayer 206 and the layer portion having index n₂ in the layer 214.

The system 204 also includes a layer means for preferential processingof polarized light of one state relative to another state, such as apolarization filter layer 216 (see previous generic description of thepolarization filter layer 307). In addition to the samples described forthe filter layer 307, a further example of the polarization filter layer216 is a birefringent material which will be described hereinafter inthe context of particular embodiments in a separate subsection. In FIG.19, the injected light 212 includes light 218 of a first polarizationand light 220 of a second polarization. The filter layer 216 theninteracts with the light 212 to preferably output the light 218 of afirst polarization state compared to the light 220 of a secondpolarization state. This filter layer 216 is disposed beyond the secondsurface 210 relative to the base layer 206, and this filter layer 216 isalso able to reflect at least part of the light 220. This reflectedlight 220 is then transmitted through both the first layer 214 and thebase layer 206 and into a medium 207 having index n₃ (such as air). Thelight 218 on the other hand is output from the system 204 on the side ofthe base layer 206 having the polarization filter layer 216. In FIG.19B, the light 218 is shown being output into a media 221 having indexn₄. In this embodiment in FIG. 19B, the relationship among indices is:

    n.sub.4 ≧n.sub.2 and

    arcsin(n.sub.2 /n.sub.1)-2Φ<arcsin(n.sub.3 /n.sub.1)<arcsin(n.sub.2 /n.sub.1)+2Φ                                          (9)

In this preferred embodiment n₂ and n₃ can be air layers with "n" beingapproximately one.

This same index relationship can apply to FIG. 19A which is a variationon FIG. 19B, but the first layer 214 of index n₂ is disposed furtherfrom the base layer 206 than the polarization filter layer 216. In theembodiment of FIG. 19B, the first layer 214 is closer to the base layer206 than the polarization filter layer 216.

In another embodiment shown in FIG. 19C, the indices are such thatEquation (10) below is followed and this results in the light 220 ofsecond polarization state continuing to undergo internal reflection,rather than exiting through the first surface 208 as shown in FIGS. 19Aand 19B. The angle of incidence made relative to the polarization filterlayer 216 decreases with each cyclic reflection. The index n₃ can thusbe made small enough such that the light 220 will decrease its anglebeyond the range where the filter layer 216 exhibits its preferredreflectivity of the light 220. Consequently, at least part of the light220 can pass through the second surface 210, but is separated in angleof output relative to the light 218 of first polarization state. In theembodiment of FIG. 19C the indices have the following relationship:

    n.sub.4 ≧n.sub.2 and arcsin(n.sub.3 /n.sub.1)<arcsin(n.sub.2 /n.sub.1)-4Φ                                          (10)

The polarization filter layer 216 most preferably outputs the light 218and reflects the light 220 when the angle of incidence is greater than:

    θ.sub.p =arcsin[1-4Φ((n.sub.1 /n.sub.2).sup.2 -1).sup.1/2](11)

When light is incident at angles less than θp, the filter layer 216 cantherefore be substantially transparent to light of both polarizationstates (i.e., the light 218 and the light 220).

In another embodiment of the invention shown in, for example, FIGS.20A-C, the system 204 includes light redirecting means, such as a lightreflector layer 222 in FIG. 20A, or more genetically, a lightredirecting layer 224 as shown in FIGS. 20B and 20C. In general for theinventions of the device 10 system 204 in FIG. 20, we can define lightredirecting means in terms of the propagation directions of light raysincident on, and departing from, the light redirecting layer 224.Consider the case of a light ray propagating parallel to a unit vectorr_(i) in an optical medium having an index of refraction n_(i). If u isa unit vector perpendicular to the redirecting layer 224 at the point oflight ray incidence and directed away from the redirecting layer 224toward the side from which the incident light ray originates, then theincident light ray interacts with the light redirecting layer 224 toproduce light rays which depart from the region of interaction. If thedeparting light rays propagate parallel to a distribution of unitvectors r_(c) in an optical medium having index of refraction n_(c),then light redirecting means includes any layer which processes theincident light ray such that the departing light ray has one of thefollowing properties with respect to incident light rays throughout theoperative angular range:

    (1) n.sub.c (r.sub.c ×u) is not equal to n.sub.i (r.sub.i ×u)(12)

for at least 25% of the departing light rays;

    (2) r.sub.c =r.sub.i -2(u·r.sub.i)u               (13)

for at least 90% of the departing light rays.

The light redirecting layer 224 can redirect light according tocondition (1) in Equation (12) if (a) the light interacts with opticalsurfaces which are rough, (b) if the light interacts with opticalsurfaces which have a different slope from the incident surface, or (c)if the redirecting layer 224 diffracts the light into appropriateangles. For example, light redirecting means according to condition (1)may be any combination of transmissive or reflective, diffusive ornon-diffusive, and prismatic or textured layer. In addition, the lightredirecting means can be a diffraction grating, a hologram, or a binaryoptics layer.

A light redirecting means which redirects light in accordance withcondition (2) of Equation (13) is a specular reflector. Examples of sucha specular reflector can be a metallic coating (e.g., the lightreflector layer 222 in FIG. 20A can be a metallic coating), amulti-layer dielectric coating or a combination of these. In each case,the internal and external surfaces are preferably smooth and mutuallyparallel.

In FIG. 20A one of the preferred embodiments includes light reflecting,redirecting means in the form of the reflector layer 222 which reflectsthe light 220. The reflector layer 222 is disposed beyond, orunderlying, the first surface 208 of the base layer 206 and preferablyis a flat, specular reflector, such as a metallic coating. Also shown isan intervening layer 223 of index n₃ disposed between the base layer 206and the reflector layer 222. This intervening layer 223 can beconsidered to be part of the base layer 206, or a separate layer,depending on the functional interaction between the base layer 206 andthe intervening layer 223. The index of refraction n₃ of thisintervening layer 223 can be adjusted to controllably affect theresulting spatial and angular distribution of the light 212 afterencountering the layer 223.

As can be seen, for example, in FIGS. 20B and 20C the light redirectinglayer 224 can be positioned at different locations, and each layer 224can also have different characteristics enabling achievement ofdifferent light output characteristics as needed for a particularapplication. Further examples of light redirecting means and uses, aswell as specific embodiments, are illustrated in the remaining figuresand will be described in detail hereinafter.

In another embodiment of the polarized light luminaire system 204, lightconverting means is included and is illustrated as a polarizationconverting layer 226 in FIGS. 21 and 22, for example. In theseillustrated embodiments, the indices have n₄ ≧n₂ and the conditions ofEquation (9) must in general be met. In these embodiments, a lightconverting means includes a layer which changes at least part of onepolarization state (such as the light 220) to another polarization state(such as the light 218, or even light 227 of a third polarization state,which can be, for example, a combination of the first and second state).

The polarization converting layer 226 has the function of changing thepolarization state to another state, such as rotating polarization by90° (π/2). Moreover, such conversion is most preferably done for obliqueincidence. As one example we describe the nature of such conversion fora uniaxial birefringent material where the index of refractionperpendicular to the optic axis is independent of direction. Manypreferred materials, such as stretched fluoropolymer films are of thistype. More general birefringent materials where the index of refractionis different in all directions can also be used following the generalmethods described herein. To understand the polarization conversionprocess, we first review the case for normal incidence.

As shown in FIG. 30E, a plate 229 of birefringent material has itstransverse axis along vector K and the optic axis is along vector I (seevectors in FIG. 30F). For a stretched birefringent film, the directionof stretch would be along vector I. Vectors I, J, K are an orthogonaltriad of unit vectors along the x,y,z axes. For normal incidence, thewave normal is along vector K. We can describe the polarization of theelectromagnetic wave by its displacement vector D. Let D' be thepolarization of the ordinary ray, and D" the polarization of theextraordinary ray. Let n' be the ordinary index of refraction, and letn" be the extraordinary index of refraction. We can orient the opticaxis of the birefringent plate 229 so that it makes an angle of 45°(π/4) to the incident polarization vector D₀. This vector has twocomponents D₀ x=(1/√2)D₀ cos ωt and D₀ y=(1/√2)D₀ cos ωt. Upon emergingfrom the birefringent plate 229, the D vector has components D₀x=(1/√2)D₀ cos(ωt-δ") and D₀ y=(1/√2)D₀ cos(ωt-δ'), where δ"=(2π /λ)n'hand δ"=(2π/λ)n"h, where h is the plate thickness. Hence the phasedifference introduced is δ"-δ"=|(2π/λ)(n"-n')|h. In particular, if theemergent light has polarization vector D at right angles to the intialpolarization vector D', we need δ'-δ"=π(or more generally δ'-δ"=(2m+1)π,where m is any integer). This means the thickness h should be chosen ash=|(2m+1)/(n"-n')|λ/2.

In summary, we choose the thickness h in accordance with the aboverelation and orient the optic axis at 45° to the incident polarization.In a preferred form of the invention such as in FIG. 26B, the lighttraverses the converting layer 226 birefringent plate 229 twice, so thatthe actual thickness should be one-half of that specified above. Inother words, the thickness is the well known λ/4 plate. Any reflectionsfrom a metallic mirror 231 introduces an additional phase shift ofapproximately π to both components and does not change the conclusions.

In an embodiment wherein the light has oblique incidence with theconverting layer 226 (see FIG. 26B), it is first necessary to show thatsplitting of the incident beam into two beams (the well-knownbirefringent effect) does not cause difficulties. The reason this is nota problem is that the two beams emerge parallel to the initialdirection, but slightly displaced from one another. The two beams arecoherent with each other and the displacement is <λ. The angularsplitting is Δθ≈tanθ_(c) ΔΔn/n where θ_(c) is the critical angle andΔn=(n"-n'),n=(n"+n')/2. The displacement is ≈hΔθ/cos θ_(c)=hΔ(n/n)tanθ_(c) /cos θ_(c). But, we will choose hΔn/cos θ≈λ/4, soautomatically the displacement is <λ and the two light beams can betreated as one.

The geometry of oblique incidence on a uniaxial form of the birefringentplate 229 is somewhat complicated, and thus to simplify matters, weintroduce the Eulerian angles as shown in FIG. 30F. The relationsbetween the (i,j.k) vector triad and the (I,J,K) ventor triad can beread from Table 3.

                  TABLE 3                                                         ______________________________________                                        I             J               K                                               ______________________________________                                        i   - sin φ sin ψ                                                                       cos φ sin ψ                                                                           sin θ cos ψ                           + cos θ cos φ cos ψ                                                           + cos θ sin φ cos ψ                           j   - sin φ cos ψ                                                                       cos φ cos ψ                                                                           sin θ sin ψ                                         - cos θ sin φ                                         - cos φ cos θ sin ψ                                                           sin ψ                                                   k   sin θ cos φ                                                                       sin θ cos φ                                                                         cos θ                                 ______________________________________                                    

Let the normal to the air/plate interface=K, the direction of theincident wave normal=k, and the optic axis of the plate 229=I. We wishto rotate the incident polarization D₀ by 90°. Since the incidentpolarization D₀ is in the interface plane, it is consistent to let D₀ bealong i₀ so that ψ₀ =π/2. The polarization D' of the ordinary ray isperpendicular to both I and k. Therefore, let D' be along i'. Now i'_(x)=0. From Table 3 we conclude that tanψ'=cot φcos θ. The polarization ofthe extraordinary ray D" is perpendicular to both D' and k. Therefore,ψ"=ψ'±π/2. We choose ψ"=ψ'-π/2, and then tanψ"=tanφ/cos θ. To achievethe desired output, we can appropriately orient the birefringent plate229. Just as in the normal incidence case, we let ψ₀ to be at 45° to theD' and D" directions. Therefore, we chose ψ'=π/2, and then tanφ=cos θ.For a typical case, where θ is close to θ_(c) ≈40°, φ≈37°. In practice,for a range of incidence angles and wavelengths one would readily adjustφ experimentally to get the most complete polarization conversion, usingthe above formulae as a starting point and guide. We next determine thethickness, h, of the birefringent plate 229. As in the case of normalincidence, the condition is: h=|(2m+1)/(n"-n')|,λ/2. However, theextraordinary index of refraction n" now depends on the angle ofincidence θ and must be read off the index ellipsoid: (1/n")² =(1/n₀)²sin² θ+(1/n_(e))² cos² θ where n₀ is the ordinary index of refractionand n_(e) is the extraordinary index of refraction. Also note thatn'=n₀. Typically, the index of refraction differences are small, <0.1and approximately, (n"-n')≈(n_(e) -n_(c))cos² θ. In addition, the lightpath length for oblique incidence is greater than that for normalincidence. The length h for oblique incidence is greater than thethickness of the plate 229 by a factor of 1/cos θ. Therefore, since theeffective index difference is reduced by cos² θ, but the path length isincreased by 1/cos θ, it follows that the thickness required for obliqueincidence is larger than for normal incidence by ≈1/cos θ. In practice,for a range of incidence angles and wavelengths one would adjust hexperimentally to obtain the most complete polarization conversion. Inpractice, for a range of incidence angels and wavelengths, one canadjust φ experimentally to obtain the most complete polarizationconversion, using the above formulae as a starting point and guide.

In another example embodiment, the conversion of light of onepolarization into another polarization state can be considered asinvolving three steps: (1) separation of different polarization statesinto substantially distinct beams at every point on the system 204, (2)polarization conversion without affecting the desired polarization and(3) light diffusion into an appropriate angular distribution withoutdepolarization of the light output.

As divided herein, a variety of methods can be used to separate thedifferent polarization states in the system 204. For example, the lowindex layer 214 can be birefringent, as shown, for example, in FIGS.31A-C. The layer 214 can be, for example, an oriented fluoropolymerconvertor layer which creates two light beams 218 and 220 of orthogonalpolarization emerging from every point along the system 204. This can beused provided two conditions are met. The first condition requires thatthe birefringence of the layer 214 is large enough to significantlyprevent overlap between the two polarized beams 218 and 220. Thiscondition is summarized by Equations (15)-(17) where S is at least 1 andpreferably greater than 4. The second condition is that the direction ofbirefringence orientation (direction of stretch) of the first layer 214is substantially parallel to the y axis.

For φp=1-1.5 degress, the birefringence must be at least 0.03-0.05 tosatisfy Equations (15)-(17). Measurements of the birefringence ofvarious commercial fluoropolymer films yielded the following data(average index, birefringence):

Tefzel 250 zh: (1.3961,0.054)

Tefzel 150 zm: (1.3979,0.046)

Teflon PFA 200 pm: (1.347,0.030)

The wedge layer 206 laminated with the 250 zh material producedjust-separated polarized beams where even the Fresnel reflected partsdid not overlap.

In another embodiment, one can achieve even greater angular separationof polarization by using a faceted redirecting layer comprised of ahighly birefringent material.

A third approach for separation of polarization states uses a sheet ofpolymeric beam splitters consisting of an alternating structure ofbirefringent/transparent layers 427 shown in FIG. 30G and H. Such anarray can rest on top of a collimated backlight 428 and polarizes byselective total internal reflection. The index of the film of polymericlayers 429 parallel to the plane of light incidence is lower than thatof a transparent layer 430, and the index perpendicular to the plane oflight incidence is closely matched to the transparent layer 430, so thatan incoming collimated light beam 431 from the backlight 428 (inclinedto the beam splitter layers 427) is split: the parallel polarized beam431 is totally internally reflected, but the perpendicular component istransmitted.

One example of this arrangement can be Mylar/Lexan layers. Mylar indexesare: (1.62752,1.6398,1.486). The Lexan index is: 1.586. The complementof the critical angle is twenty degrees; therefore, the beam splitterlayer 427 will function as long as the complement of the incidence angleis less than twenty degrees (in the Lexan). However, at glancing angles,Fresnel reflection causes reduction in the degree of polarization. Forexample, for thirteen degrees the Fresnel reflected perpendicularcomponent is 9%.

Another example of this arrangement of the layer 427 is uniaxialNylon/Lexan. Nylon indexes are: (1.568,1.529,1.498). Here there are twocritical angles, the complements of which are nine and nineteen degreesfor perpendicular and parallel, respectively. So, the obliquity must beinside this angular range for polarization to be operative. Taking thesame case for Fresnel reflection as for Mylar (thirtee degree angle),the Fresnel reflected perpendicular component is only 5%, because theindex matching is better.

For either of these examples, each beam splitter layer 427 needs to havethe appropriate aspect ratio such that all rays of the beam 431 haveexactly one interaction with the film/Lexan interface.

In one embodiment, once the light of different polarization states isseparated into two orthogonally polarized beams at every position alongthe backlight 428, there must be a means of converting the undesiredpolarization to the desired one, such as the polarization convertorlayer 346 in FIG. 31C and 429 in FIG. 30G.

One method of performing the polarization conversion is by analternating waveplate combined with a lens or lens array. In the singlelens method, a light beam 218 and 220 will fall upon lenses focused totwo nonoverlapping strips of light of orthogonal polarization at thefocal plane. The alternating wave plate acts to rotate the polarizationof only one of the beams (220) by ninety degrees, the emergent lightwill be completely converted to light 218. This can be effected by thepresence of a half-wave retarder placed to capture only the light 220 ofone polarization. This has been demonstrated visually with a large lens,a plastic retardation plate, and Polaroid filters (Polaroid is aregistered trademark of Polaroid Corporation).

In a second approach using a lenticular array, one uses a thin sheet oflenses and an alternating waveplate structure (with the frequency equalto the lens frequency), where the retardation changes by 180 degrees foreach lens. For a lenticular array 1 mm thick, each image can be of theorder of 5 thousandth of an inch in size so the registration of thelenticular array with the waveplate would have to be exact enough toprevent stack-up errors of less than one thousandth of an inch.

Another method of performing the polarization conversion is by use of adouble Fresnel rhombus ("DFR") which is another embodiment of aconverting layer, such as the layer 346 in 31C and new number 427 in30G. The DFR avoids registration problems by selectively retardingaccording to angle instead of position. Such a DFR causes the light offirst polarization state to suffer from total internal reflection eventscorresponding to 4×45°=180° of phase shift, while the other polarizationstate light is only transmitted, so that the output light is completelypolarized to the light of first polarization in one plane in the end.The DFR can be constructed, for example, by having four acrylic or Lexanfilms each embossed with 45 degree prisms, all nested. For the DFR tocause retardation the two orthogonal plane polarized beams L and R (by a1/4-wave plate). If the L is transmitted by the DFR then the R beam willget converted to the L beam by the DFR. Finally the L beam is convertedto plane polarized by another 1/4-wave plate, the orientation of whichdetermines the final plane of polarization.

In a preferred embodiment shown in FIG. 21A, the converting layer 226 isdisposed on the opposite side of the base layer 206 relative to thepolarization filter layer 216. In the embodiment of FIG. 21B, theconverting layer 226 is disposed on the same side as the polarizationfilter layer 216. As can be seen by reference to FIGS. 21A and B, theconverting layer 226 can even convert the light 218 and 220 to the lightof 227 of another third polarization state. This light 227 can be, forexample, the light of a third polarization state or even a variation on,or combinations of, the first or second polarization states discussedhereinbefore. The resulting light polarization is dependent on theresponse characteristics of the converting layer 226. The convertinglayer 226 can therefore be designed to respond as needed to produce alight of desired output polarization state; and in combination withappropriate positioning of the layer 226, one can produce an outputlight in the desired direction having the required polarizationcharacteristics.

In another form of the invention illustrated in FIGS. 22A-E, theconverting layer 226 is utilized for other optical purposes. FIGS. 22,23, 24E-F, 25-27, 28A and C, and 29 all illustrate use of the convertinglayer 226 to change the light 220 of the second polarization state tothe light 218 of the first polarization state. In addition, the elementsof the luminaire system 204 are arranged such that the light beingprocessed will pass through, or at least encounter, one or more of thepolarization filter layer 216 at least once after passing through theconverting layer 226. For example, in the case of processing the light220, the arrangement of elements enables return of the light 220 to passthrough the polarization filter layer 216 after passing through theconverting layer 226. In some instances, the light 220 can encounter thepolarization filter layer 216 two or more times before being output asthe light 218 of the first polarization state. FIGS. 22A-E illustrateexamples of a variety of constructions to achieve a desired output. InFIG. 22A, after the light 212 encounters the polarizing filter layer216, the reflected light 220 passes through the converting layer 226,and is converted to the light 218. The light is then returned to thepolarization filter layer 216 via internal reflection. In addition, inFIG. 22B, the light 220 also passes through the converting layer 226, isconverted to the light 218, and is then returned again to the filterlayer 216 after internal reflection. In these cases, n₃ is low enoughsuch that the relationship among n₁, n₂ and n₃ in Equation (10) is met.

In the embodiments of FIGS. 22C-E, a redirecting means in the form ofthe light reflector layer 222 is added to return the light 220 to thepolarization filter layer 216. As described hereinbefore for theembodiment of FIG. 20A, the intervening layer 223 has an index ofrefraction n₃ which can be adjusted to affect the spatial and angulardistribution of light encountering the layer 224. In a preferred form ofthe invention shown in FIGS. 22C-E, the layers of index n₂ and n₃ caninclude air gaps, and in the most preferred form of the invention thelayers of index n₂ are air gaps.

FIGS. 24A-F illustrate a sequence of constructions starting with use ofone of the polarization filter layer 216 in FIG. 24A and continuingconstruction of more complex forms of the luminaire system 204. In FIGS.24C-F, there is added one or more of the light redirecting layer 224, atleast one liquid crystal display ("LCD") layer 230 and light matchingmeans, such as a matching layer 232. The matching means acts to convertthe light output by the assembly of the other layers to a particularpolarization state preferred by a target device or additional layer,such as the LCD layer 230. The matching layer 232 is thus a special caseof the converting layer 226.

In FIGS. 23A-C are illustrated other forms of the polarized lightluminaire system 204 in combination with the LCD layer 230. In onegeneral form of the embodiment of FIG. 23A, a layer 234 is included. Inmore particular forms of the inventions, for example as in FIG. 23, thepreferred value of n₂ is about 1 (see, for example, FIGS. 23B and C). Incertain forms of FIG. 23A, n₂ >1 can also be utilized. Alternatively,preferably choices for the relationship among indices of refraction areset forth in Equation (9) and (10).

Further examples of preferred embodiments are shown in FIGS. 26A and B,and in FIG. 26A is included a cold cathode fluorescent tube ("CCFT")light source 236. This embodiment further includes an angle transformerlayer 238 which operates to change the angular distribution of thelight. This angle transformer layer 238 can, for example, change thedistribution in the xz-plane to control the spatial uniformity of lightoutput from the device 10. In the preferred embodiment, the distributionof the output light 250 is substantially uniform in its spatialdistribution over at least 90% of the output surface. In addition, theangular distribution of the light 212 in the xz-plane is approximately±θ_(max) with respect to the normal to the back surface 211, where##EQU9## and the back surface 211 is about perpendicular to at least oneof the first surface 208 and the second surface 210. The angletransformer layer 238 can be a tapered light-pipe section, a compoundparabolic concentrator (a "CPC"), a microprismatic film (FIG. 28C) aroughened-surface layer, or a hologram. The angle transformer layer 238is most preferably optically coupled to the base layer 206 without anintervening air gap. The angle transformer layer 238 can also operate tochange, and preferably narrow, the light distribution in the yz-plane toimprove brightness, LCD image quality, and viewer privacy as well. Inaddition, in FIG. 26A, an output diffuser layer 248 has been addedbefore the LCD layer 230 to broaden the angular distribution and enhanceuniformity of output light 242 provided to the LCD layer 230.

In another preferred embodiment of FIG. 26B, a CPC 239 is coupled to alight source 244 operating to help maintain output light 250 within theproper angular distribution in the xz plane. In addition, one cancontrol the range of angular output by use of a light redirecting means,such as a prismatic redirecting layer, such as the layer 246, using flatprismatic facets, such as the facets 247. See, for example, this type oflayer and prismatic facets in FIGS. 28C, D and E and FIGS. 29A and B andthe description in detail provided hereinafter. This embodiment as shownin FIG. 28E refers to the prismatic layer 251 and facets 253, and thisembodiment also adds after the LCD layer 302 a light diffuser layer 304for broadening light distribution in a specific plane. In a mostpreferred form of this embodiment, for example, shown in FIG. 28E, thelight 242 is directed to pass through the LCD layer 302 within a narrowangular range in the xz-plane. The elements of the luminaire system 204are therefore constructed to assist in providing transmission of thelight 242 through the LCD layer 302 at an angle where the image formingproperties are optimized. With the diffuser layer 304 positioned on theother side of the LCD layer 302 relative to the base layer 206, thediffuser layer 304 can broaden the angular distribution of viewer outputlight 250 without diffusing the light 250 in the xy-plane. For example,the diffuser layer 304 can be a "parallel" diffuser which can take theform of a holographic diffuser or lenticular diffuser with groovessubstantially parallel to the y-axis. Viewers at a wide range of anglescan then see the image which is characteristic of the optimal angle forthe light 242 which is subsequently transmitted through the LCD layer302 to form the light 250. Example configurations utilizing this form ofgeneral construction are thus shown in FIGS. 28D and E and FIGS. 29A andB. Further, FIGS. 28D and E and FIG. 29A also include a transversediffuser layer 252 which diffuses the output light 242 provided to theLCD layer 302 only in the xy-plane in order to improve uniformitywithout broadening the distribution of the light 242 in the xz-plane.For example, the transverse diffuser 252 can be a holographic diffuseror a lenticular diffuser with grooves substantially parallel to thez-axis. Further details will be described hereinafter.

In FIGS. 27A and B are additional preferred embodiments wherein thefirst layer means of index of refraction n₂ is most preferably not air.These embodiments show different examples of the light redirecting layer224. Further, in FIG. 27A medium 254 having index n₃ need not be air,but the various indices of the system 204 must meet the requirements ofEquation (10) to achieve the total internal reflection illustrated. InFIG. 27B the medium 254 is air, the light redirecting layer 224 hascurved facets 256, and the light 245 is focused within a preferredviewing zone 258.

The embodiments of FIGS. 28 and 29 preferably utilize an air gap layer260 as the first layer means. The layer 260 enables light to enter thelayer 260 after the light 212 has achieved an angle of incidence lessthan the critical angle θc characteristic of an interface between thebase layer 206 and the air gap layer 260. The embodiment of FIG. 28Bincludes a first redirecting layer 262 between the base layer 206 and adiffuser layer 264 and a second redirecting layer 265 on the other sideof the base layer 206. This first redirecting layer 262 includesrefracting/internally reflecting prisms 266 while the second redirectinglayer 265 includes refracting prisms 268. Two of the polarization filterlayer 216 are disposed either side of the base layer 206, eachtransmitting the appropriate light 218 or 220 which is passed throughthe associated light redirecting layer, 262 and 265, respectively. InFIG. 28C is a more preferred embodiment wherein the light redirectinglayer 246 comprises a refracting/internally reflecting layer having therelatively small prisms 247. The surface angles of each of the prisms247 can vary across the illustrated dimension of the redirecting layer246 in a manner described hereinbefore. This variation in angle enablesfocusing different cones of light coming from the prisms 247 onto thepreferred viewing zone 258 (see FIG. 27B). The light reflector layer 222can be a metallic coating as described hereinbefore.

The reflector layer 222 can be applied to the converting layer 226 byconventional vacuum evaporation techniques or other suitable methods.The other layers, such as the redirecting layer 246 can be formed bycasting a transparent polymeric material directly onto the matchinglayer 232 (see FIGS. 24C-F and 28C and D). The polarization filter layer216 can likewise be manufactured by conventional methods, such asdeposition of multiple thin layers directly onto the base layer 206.Also included is an angle transformer layer 274 coupled to the backsurface 211 (see FIG. 28C). This angle transformer 274 includes prisms276 which broaden the angular distribution of input light 212 to thebase layer 206 to help provide a more spatially uniform form of theoutput light 218 to the LCD layer 230. Other forms of the angletransformer layer 274 can be a roughened layer and a hologram (notshown) coupled to the back surface 211 (or other input surface) withoutan intervening air gap.

In the preferred embodiment of FIG. 28D, a first prismatic lightredirecting layer 249 is disposed between the base layer 206 and thepolarization filter layer 216. This redirecting layer 249 reduces theangle of incidence of light 280 incident on the polarization filterlayer 216. A second prismatic light redirecting layer 282 then redirectslight 284 output from the filter layer 216 to an LCD layer 302 with apost diffuser layer 304, operable as a parallel diffuser as describedhereinbefore. This embodiment further includes the CCFT light source 236with a reflector 290 having a position following at least a portion ofan involute of the light source 236 inner diameter. Another portion ofthe reflector 290 directly opposite the back surface 211 is convexlycurved or bent.

In the preferred embodiment of FIG. 28E a light redirecting layer 251comprises refracting micro prisms 253. A polarization filter layer 296is disposed adjacent a converting layer 298, and the transverse diffuserlayer 252 is positioned between the redirecting layer 251 and the LCDlayer 302. A parallel diffuser 304 is disposed on the light output sideof the LCD layer 302 with the light 242 directed through the LCD layer302 at a preferred angle to optimize output light 301 for bestimage-forming quality of the LCD layer 302 (contrast, color fidelity andresponse time).

The embodiments of FIGS. 29A and B show some of the advantages of someforms of the invention over a conventional LCD polarizer system 304shown in FIG. 30A. In FIG. 30A, a prior art backlight 306 emits light308 of both polarizations in nearly equal proportions. A typical priorart LCD layer arrangement 310 includes a first form of polarizationfilter 312 and a second form of polarization filter 314 with the liquidcrystal layer 316 sandwiched therebetween. In this LCD layer arrangement310, the first polarization filter 312 must provide a high polarizationratio, that is, it must have an extremely low transmission of light ofthe second polarization state which is unwanted for input to the liquidcrystal layer 316 in order for the LCD layer arrangement 310 to provideadequate LCD contrast. In practice, the polarization filter 312 has ahigh optical density for the desired light of the first polarizationstate as well. The resulting losses therefore further degrade the LCDlight transmission and image output. In contrast to this prior artarrangement 310, the invention provides a much higher percentage oflight which is preferred by the LCD layer arrangement 316 thereby makinguse of a substantial portion of the light of the unwanted secondpolarization and also minimizing loss of the desired light of the firstpolarization state.

In the embodiment of FIG. 28A this advantageous processing of the light218 and the light 220 for the LCD layer 316 is accomplished bypositioning the converting layer 226 adjacent the base layer 206.Disposed adjacent the converting layer 226 is the polarization filterlayer 216. The light redirecting layer 224 includes curvedmicroprismatic facets 318 to broaden the angle of light distribution inthe xz plane and improve the uniformity of light distribution outputfrom the luminaire system 204. A transverse diffuser 320 is preferablylaminated to the light redirecting layer 224 or can be formed onopposite sides of a single polymeric layer (not shown). The polarizingfilter layer 216 can be laminated or is disposed directly onto theconverting layer 226 which in turn is laminated or deposited directlyonto the first surface 208.

In the preferred embodiment of FIG. 29A the advantageous processing ofthe light 218 and the light 220 for the LCD layer 302 is accomplished byusing a first polarization filter layer 324 and a second polarizationfilter layer 322. The first filter 324 can, however, have a relativelylow polarization ratio compared to the prior art polarization filter312. For example, the polarization falter layer 324 can have a lower dyeconcentration than the prior art filter 312. This difference enableshigher LCD light transmission and improved image-forming propertiesdescribed hereinbefore. This preferred embodiment utilizes a postdiffuser layer 328 which is coupled to an LCD system 330 (thecombination of the layer 324, the liquid crystal layer 302 and the layer322). Preferably the post diffuser layer 328 is laminated to, orintegrally formed with, the second polarization filter layer 322.

In the preferred embodiment of FIG. 29B, the advantages are achieved byusing only one polarization filter layer 248 which results in reducedcost for the luminaire system 204 and increased light transmission. Inthis embodiment the light output through the matching layer 232 ispreferably at least 90% composed of light 218 of the LCD preferredpolarization state. A coupled angle transformer 334 coupled to the backsurface 211 reduces the angular width of light distribution in the yzplane, and this reduced angular distribution further improves quality ofthe output light 250 making up the LCD image from the luminaire system204.

Birefringent Layers in Luminaire Systems

A birefringent material can be used to advantage in the polarized lightluminaire system 204 discussed hereinbefore. In the embodimentillustrated in FIG. 31A, the first layer 214 can be a birefringentmaterial of index n₂ with two different optical indices n₂α and n₂β forthe light 212 of two different polarization states "a" and "b", bothindices being less than one. This light 212 encounters the layer 214near the respective critical angles for these two polarization states,

    θ.sub.cα =arcsin(n.sub.2α /n.sub.1)      (15)

and

    θ.sub.cβ =arcsin(n.sub.2β /n.sub.1)        (16)

The conditions of Equation (10) must be satisfied for n₂ equal to bothn₂α and n₂β, independently. The light 212 of both polarization statesdecreases its angle of incidence by an angle 2φ for each cyclicreflection from the first surface 208 and the second surface 210 asdescribed previously. In this embodiment n₂α >n₂β and therefore θ_(c)α>θ_(c)β. As the incidence angle for both polarization states decreases,the light 212 of both polarization states can encounter the interfacewith the birefringent first layer 214 with the light having an incidenceangle less than the first critical angle θcα, but exceeding the secondcritical angle θ_(c)β. Therefore, light 218 of the first polarizationstate is at least partially transmitted through the birefringent firstlayer 214, while the light 220 of the second state is preferentiallyreflected by total internal reflection. This reflected second-statelight 220 and the residual first-state light 218 continue to decreasetheir angles of incidence with successive reflections. The light 218 ofthe first polarization state is transmitted at each successive encounterwith the interface between the first layer 214 and the base layer 206.The light 220 of the second state continues to undergo total internalreflection at this interface until its angle of incidence becomes lessthan the second critical angle θcβ, at which point this second-statelight 220 also is at least partially transmitted through thebirefringent first layer 214. By virtue of this mechanism and of thedifference in indices n₂α and n₂β, the light exiting the birefringentfirst layer 214 has a different angle distribution for the twopolarization states "a" and "b".

Birefringent materials can in general include crystalline materialshaving an anisotropic index of refraction. A preferred material is astretched polymeric film such as stretched fluorinated film. Thestretching orients the film and makes the index of refraction differentalong that direction. Elsewhere we give birefringence values of thesestretched fluoropolymer film with Δn ranging from 0.030-0.054. Otherfilms are PVA (Polyvinylalcohol). Polypropylene, Polyolefin or evenPolyester (Mylar). Mylar is actually biaxial, but may still be used torotate polarization. More traditional uniaxial birefringent materialsare: Calcite and Quartz. These are not as practical as the stretchedfilms. In practice the two polarization states are well-separated onlyif the two indices are sufficiently different. This condition may beexpressed as,

    θ.sub.ca ≧θ.sub.cβ sφ          (17)

where s must be at least 1 and is preferably greater than four. Thiscondition may be achieved, for example, using uniaxially orientedfluoropolymer material for the birefringent layer, acrylic polymer forthe base layer 206 and reasonable values of φ (between one andone-and-a-half degrees is typical for notebook computer LCDbacklighting).

FIG. 31B is like FIG. 31A, but the redirecting layer 224 has been added;and the preferred embodiment uses air for the layer 207 having index n₃.The light 218 and the light 220 are output from the system 204 atdifferent angles.

FIG. 31C illustrates another variation on FIGS. 31A and B, but theredirecting layer 224 comprises a flat faceted reflective layer 340. Thelight 218 and also the light 220 are directed to a converting layer 346which transmits the light 218 without substantially changing itspolarization state; however, the converting layer 346 does convert thelight 220 to the light 218 of the desired first polarization state. Theconverting layer 346 shown in FIG. 31C has a construction that operatesto convert the light polarization only within the angular range occupiedby the light 220. The converting layer 346 thus utilizes theschematically illustrated angular separation of the light 218 and thelight 220 to carry out the conversion of the light 220 to the light 218without converting the light 218 to the light 220.

In the embodiments of FIGS. 31D and E, the reflected form of the light220 is returned to the interface of the base layer 206 with thebirefringent first layer 214. This is accomplished by virtue of totalinternal reflection of the light 220 together with passing at leasttwice through the converting layer 346, which results in at leastpartially converting the light 220 into the light 218 of the firstpolarization state. Since this light 218 has an incidence angle lessthan the first critical angle θ_(c)α, the light 218 is transmittedthrough the interface between the base layer 206 and the first layer214. This light 218 can then be reflected or transmitted by theredirecting layer 224, depending on the particular nature of theredirecting layer 224. The alternatives of transmitted and reflectedlight are shown in phantom in FIGS. 31D and E. Further, in theembodiment of FIG. 31D, the converting layer 346 is on the same side ofthe base layer 206 as the birefringent first layer 214. The convertinglayer 346 is also disposed between the base layer 206 and thebirefringent first layer 214. The embodiment of FIG. 31E shows anothervariation on FIG. 31D with the converting layer 226 and the birefringentfirst layer disposed on opposite sides of the base layer 206.

In the embodiment of FIG. 3IF the system 204 is similar to theembodiment of FIG. 31D, but the redirecting layer 224 comprises a layerof facets 311. In the embodiment of FIG. 31G, the system 204 furtherincludes the LCD layer 302, the matching layer 232, and the diffuserlayer 304 is disposed in a spatial position after the light 218 haspassed through the LCD layer 302. The redirecting layer 224 comprisesthe layer of microprisms 251 having flat faces and a metallic coating342 for high light reflectivity. Also shown is the angle transformerlayer 238 to control the spatial distribution of the light 253 outputfrom the system 204. The embodiment of FIG. 31H is similar to theembodiment in FIG. 31G, but the system 204 uses curved facets 345 forthe redirecting layer 224 with facet angles adjusted at differentspatial locations to focus the output light 250 onto a preferred viewingzone. The angle transformer 238 is illustrated as a CPC.

Light Diffuser After LCD Layer Processing

In the embodiments shown in FIGS. 12N and 12O the LCD display 216 or 236provides an output light to the viewer. In a further improvement ofthese embodiments a post diffuser layer 350 is disposed in the path ofthe light 250 output from the LCD layer 302 (see FIG. 32A and B). In thepreferred embodiments shown in these figures, the general operation issimilar to the embodiments illustrated in FIGS. 26B, 28D and E; 29A andB and 31G, but without any of the polarization filter layers 216. Asdescribed hereinbefore, it is advantageous to provide light to the LCDlayer 302 in a collimated angular range, preferably substantiallyperpendicular to the LCD layer 302 to optimize the image outputtherefrom. The use of the post diffuser layer 350 allows the outputlight 253 to provide an image to viewers over a wide angular rangewithout compromising light contrast and color fidelity.

One aspect which is preferably controlled in a system including the postdiffuser layer 350 is the width in the xz-plane of the angulardistribution transmitted through the LCD layer 302. The output angulardistribution preferably has a full width less than ##EQU10## and a fullwidth less than half of this value is even more preferred. In thisequation Δθ_(pd) is in radians, n_(LCD) is the average index within theLCD layer 302, l is the repetition period of display pixel rows in thez-direction, and d is the thickness of the LCD layer 302. For a typicalLCD used in notebook computers, n_(LCD) is approximately 1.5, l=0.3 mm,and d=3 mm. For this example, Δθ_(pd) is preferably less than 18degrees, and a full-width of nine degrees or less is even morepreferred. By comparison, Equation (8) can be used to calculate theoutput angular width of the current invention using a flat-facetprismatic redirecting layer, such as is shown in FIG. 32A (layer 359) orin FIGS. 28B (layer 262). For a typical notebook computer backlightingsystem, Φ=1.3 degrees and n=1.49. In this example, Equation (8) gives anoutput angular distribution of eighteen degrees.

FIG. 32A shows a preferred arrangement of the system 204 having aparallel form of the post diffuser 350 disposed overlying the LCD layer302. Also included is a holographic angle transformer 364 disposed onthe back surface 211.

In another embodiment shown in FIG. 32B a refracting/internallyreflecting layer 360 includes curved facets 362 in order to narrow theangular distribution in the xz-plane of light 364 directed through theLCD layer 302, and thereby to improve image quality by reducing parallaxat the post diffuser layer 350. The embodiment has the curved reflectingfacets 362, but flat refracting facets can achieve the desired functionas well, as shown in FIG. 32C. In either case, the curved facets 362preferably have a focal length less than the repetition period betweeneach of the facets 362. The angular distribution in the xz-plane ispreferably narrowed beyond the width given in Equation (8), and is mostpreferably narrowed beyond the width given in the equation above. Inaddition, the facet angles of the redirecting layer 224 are arranged tofocus the light output from different portions of the system 204 onto apreferred viewing zone. This figure also shows the micro-prismaticangle-transforming layer 274.

In FIG. 32C is shown a variation on the embodiment of FIG. 32B. In thesystem 204 an LCD layer arrangement 370 differs from the prior art LCDlayer arrangement 310 illustrated in FIG. 30. In particular, a parallellight diffuser layer 372 (such as a holographic diffuser) is disposedbetween the LCD layer 302 (layer 316 in FIG. 30) and the secondpolarization filter layer 322 (layer 314 in FIG. 30). This arrangementenables the second polarization filter layer 322 to reduce the glarewhich can otherwise be caused by ambient light being reflected by thediffuser layer 372. FIG. 32C further shows a light redirecting layer 374having curved refracting facets 376 which perform the same anglenarrowing function as the curved reflecting facets 362 shown in FIG.32B.

While preferred embodiments of the inventions have been shown anddescribed, it will be clear to those skilled in the art that variouschanges and modifications can be made without departing from theinvention in its broader aspects as set forth in the claims providedhereinafter.

We claim:
 1. An optical device for operating on light from a source andfor selectively outputting light to a viewer, comprising:a base layerhaving a wedge-shaped cross-sectional area and having an optical indexof refraction n₁, and a first and a second surface converging to defineat least one angle of inclination φ, said base layer further including aback surface spanning said first and second surfaces, and the lightexiting said base layer when the light being reflected therein decreasesits angle relative to the normal to at least one of said first andsecond layer surfaces and achieves an angle of incidence less than acritical angle θ_(c) relative to the normal; first layer means includinga layer having index n₂ less than n₁ disposed beyond said second surfacerelative to said base layer and for enabling light to enter said firstlayer means after output from said base layer when the light in saidbase layer achieves the angle of incidence less than the critical angleθ_(c) characteristic of an interface between the base layer and saidlayer of index n₂ ; and second layer means for preferably outputtinglight of a first polarization state compared to a second polarizationstate, said second layer means being disposed beyond said second surfacerelative to said base layer and said second layer means further able toreflect at least part of the light having the second polarization state.2. The optical device as defined in claim 1 wherein at least one of saidfirst and second layer means comprises a light converting means for atleast partially changing light of one polarization to anotherpolarization.
 3. The optical device as defined in claim 1 furtherincluding light redirecting layer means for changing the angle of lightfor output to the viewer.
 4. The optical device as defined in claim 3wherein said light redirecting means further comprises means forcontrolling angular range of light output.
 5. The optical device asdefined in claim 3 further including converting means for at leastpartially changing light of one polarization to light of anotherpolarization.
 6. The optical device as defined in claim 5 wherein saidconverting means comprises a birefringent layer.
 7. The optical deviceas defined in claim 1 further including light converting means forchanging light of the second polarization state to light of the firstpolarization state.
 8. The optical device as defined in claim 7 whereinsaid second layer means is disposed on the opposite side of said baselayer relative to said converting means.
 9. The optical device asdefined in claim 7 further including light redirecting means forcontrolling angular range of light to be output from said device, saidsecond layer means being disposed on the same side of said base layer assaid light redirecting means and also closer to said base layer thansaid light redirecting means.
 10. The optical device as defined in claim1 further including light redirecting means for controlling angularrange of light to be output from said optical device.
 11. The opticaldevice as defined in claim 1 wherein said base layer includes anadditional layer coupled thereto.
 12. The optical device as defined inclaim 1 further including at least one intervening layer disposedbetween said base layer and said first layer means, said interveninglayer allowing transmission of at least part of the light and saidintervening layer wherein said first layer means is comprised of atleast an air gap.
 13. The optical device as defined in claim 1 whereinsaid first layer means is disposed one of (a) further from said secondsurface than said second layer means and (b) nearer said second surfacethan said second layer means.
 14. The optical device as defined in claim1 further including a liquid crystal display disposed adjacent saiddevice with light redirected to said display by said optical device. 15.The optical device as defined in claim 1 wherein said first layer meansincludes an air gap.
 16. An optical device for operating on light from asource and for selectively outputting light to a viewer, comprising:abase layer having a wedge-shaped cross-sectional area and having anoptical index of refraction n₁, and a first and a second surfaceconverging to define at least one angle of inclination φ, said baselayer further including a back surface spanning said first and secondsurfaces, and the light exiting said base layer when the light beingreflected therein decreases its angle relative to the normal to at leastone of said first and second layer surfaces and achieves an angle ofincidence less than a critical angle θ_(c) relative to the normal; andlayer means coupled to said base layer for preferably outputting lightof a first polarization state compared to a second polarization state,said layer means further able to reflect at least part of the lighthaving the second polarization state.
 17. The optical device as definedin claim 16 wherein said layer means includes a layer with index ofrefraction n₂ less than n₁.
 18. The optical device as defined in claim17 wherein said layer means is coupled directly to said base layer. 19.The optical device as defined in claim 17 wherein said layer meansincludes an air gap.
 20. The optical device as defined in claim 17wherein said layer means comprises light converting means for at leastpartially changing light of one polarization to light of anotherpolarization and also includes light redirecting layer means foroperating on the light to control the angular output range of the lightto the viewer.
 21. The optical device as defined in claim 20 whereinsaid light converting means and said light redirecting layer means aredisposed adjacent one another.
 22. The optical device as defined inclaim 20 wherein said light converting means comprises a birefringentlayer.
 23. The optical device as defined in claim 22 wherein saidbirefringent layer is disposed on one side of said base layer comparedto said light redirecting layer means.
 24. The optical device as definedin claim 17 wherein said layer means comprises a birefringent layer,converting means for changing light of the second polarization state tolight of the first polarization state and light redirecting layer meansfor operating on the light to exert control over angular output range ofthe light to the viewer.
 25. The optical device as defined in claim 24wherein said light redirecting layer means is disposed further from saidbase layer than said birefringent layer and said converting means. 26.The optical device as defined in claim 25 wherein said converting meansis disposed closer to said base layer than said birefringent layer. 27.The optical device as defined in claim 17 wherein said layer meansfurther provides light of the first polarization state being transmittedthrough said first surface of said base layer and part of the light ofthe second polarization state being transmitted through said secondsurface of said base layer.
 28. An optical device for operating on lightfrom a source and for selectively outputting light to a viewer,comprising:a base layer having a wedge-shaped cross-sectional area andhaving an optical index of refraction n₁, and a first and second surfaceconverging to define at least one angle of inclination φ, said baselayer further including a back surface spanning said first and secondsurfaces, and the light exiting said base layer when the light beingreflected therein decreases its angle relative to the normal to at leastone of said first and second surfaces and achieves an angle of incidenceless than a critical angle θ_(c) relative to the normal; first layermeans including an air gap disposed beyond said second surface relativeto said base layer and for enabling light to enter said air gap afteroutput from said base layer when the light in said base layer achievesthe angle of incidence less than the critical angle θ_(c) characteristicof an interface between the base layer and said air gap, second layermeans for preferably outputting light of a first polarization statecompared to a second polarization state, said second layer means alsodisposed beyond said second surface relative to said base layer and saidsecond layer means further enabling reflection of at least part of thelight of the second polarization state; and light redirecting means foroperating on light having passed through said base layer to enable thelight to be output from said device.
 29. The optical device as definedin claim 28, wherein said redirecting means includes means for operatingon the light having passed through said second layer means to controlangular output of the light.
 30. The optical device as defined in claim29 wherein said light redirecting means operates on said reflected lightand redirects the reflected light toward said second layer means. 31.The optical device as defined in claim 30 wherein said light redirectingmeans is disposed beyond said top surface relative to said base layer.32. The optical device as defined in claim 31 wherein said lightredirecting means includes at least a reflecting layer.
 33. An opticaldevice for operating on light from a source and for selectivelyoutputting light to a viewer, comprising:a base layer having awedge-shaped cross-sectional area and having an optical index ofrefraction n₁, and a first and second surface converging to define atleast one angle of inclination φ, said base layer further including aback surface spanning said first and second surfaces, and the lightexiting said base layer when the light being reflected therein decreasesits angle relative to the normal to at least one of said first andsecond surfaces and achieves an angle of incidence less than a criticalangle θ_(c) relative to the normal; first layer means comprising an airgap layer disposed at least beyond said first surface relative to saidbase layer and for enabling light to enter said first layer means afteroutput from said base layer when the light in said base layer achievesthe angle of incidence less than the critical angle θ_(c) characteristicof an interface between said base layer and said air gap layer; andsecond layer means for preferably outputting light of a firstpolarization state compared to a second polarization state, said secondlayer means disposed beyond said second surface relative to said baselayer and said second layer means including an air gap and furtherenabling reflection of at least part of the light having the secondpolarization state.
 34. The optical device as defined in claim 33further including a redirecting layer overlying said first layer, saidredirecting layer transmitting light toward the viewer over a controlledangular range.
 35. The optical device as defined in claim 33 whereinsaid second layer means further comprises converting means for changingat least part of the light of the second polarization state to providelight of the first polarization state for output to the viewer.
 36. Theoptical device as defined in claim 35 wherein said second layer meansconverts light from the second to the first polarization state with atleast a ten percent efficiency.
 37. The optical device as defined inclaim 35 wherein said means for changing polarization has an index ofrefraction n>1.4, thickness>5 μm and is birefringent such that Δn>0.05.38. The optical device as defined in claim 37 wherein said birefringentsecond layer means is selected from the group consisting of polyester,acrylic and polycarbonate materials.
 39. The optical device as definedin claim 33 wherein said second layer means causes less than a tenpercent change of angular distribution of the light output to the viewerfor light incident at an angle x relative to the surface normal wherein,##EQU11## where n₁ =index of refraction of said base layer and n₂ =indexof refraction of said first layer means.
 40. The optical device asdefined in claim 33 further including third layer means disposed betweensaid base layer and the viewer, said third layer means for convertinglight of one polarization state to another polarization state.
 41. Theoptical device as defined in claim 40 wherein said third layer meansfurther includes a liquid crystal layer electrically activated forconverting the polarization of light passing therethrough from onepolarization state to another polarization state.
 42. The optical deviceas defined in claim 33 further including an air gap layer between saidliquid crystal layer and a separate layer portion forming said thirdlayer means, for converting light from the first to the secondpolarization state.
 43. The optical device as defined in claim 33wherein said second layer means is transparent to light having both thefirst and second polarization states.
 44. The optical device as definedin claim 33 wherein said birefringent second layer means comprises atleast one of a biaxially oriented layer and a uniaxially oriented layerof birefringent material.
 45. The optical device as defined in claim 33wherein the light comprises at least one of linearly polarized light,circularly polarized light and elliptically polarized light.
 46. Anoptical device for operating on light from a source and for selectivelyoutputting light to a viewer, comprising:a base layer having awedge-shaped cross-sectional area and having an optical index ofrefraction n₁, and a first and a second surface converging to define atleast one angle of inclination φ, said base layer further including aback surface spanning said first and second surfaces, and the lightexiting said base layer when the light being reflected therein decreasesits angle relative to the normal to at least one of said first andsecond surfaces and achieves an angle of incidence less than a criticalangle θ_(c) relative to the normal; first layer means including an airgap at least one of disposed beyond said first surface and beyond saidsecond surface relative to said base layer and having an optical indexof refraction n₂ for allowing transmission of light received from saidbase layer; and converting layer means for changing light of onepolarization state to another polarization state, said converting layermeans disposed at least one of overlying and underlying said base layer.47. The optical device as defined in claim 46 further including areflecting layer beyond said second surface relative to said base layerfor reflecting light of the second polarization state received from saidbase layer.
 48. The optical device as defined in claim 47 furtherincluding means beyond said reflecting layer relative to said base layerfor rotating light of the second polarization state to provide light ofthe first polarization state for output to the viewer.
 49. The opticaldevice as defined in claim 48 further including third layer meansdisposed between said first layer and the viewer, said third layer meansfor converting light of the first polarization state to light having anLCD preferred polarization state.
 50. The optical device as defined inclaim 49 wherein said third layer means comprises a birefringent layer.51. The optical device as defined in claim 46 further including lightredirecting means at least one of overlying and underlying said firstlayer means, said redirecting means for selectively redirecting lightoutput from said first layer means.
 52. The optical device as defined inclaim 46 further including second layer means for preferablytransmitting light of one polarization relative to another.
 53. Anoptical device for operating on light from a source and for selectivelyoutputting light to a viewer, comprising:a base layer having awedge-shaped cross-sectional area and having an optical index ofrefraction n₁, and a first and a second surface converging to define atleast one angle of inclination φ, said base layer further including aback surface spanning said first and second surfaces, and the lightexiting said base layer when the light being reflected therein decreasesits angle relative to the normal to at least one of said first andsecond surfaces and achieves an angle of incidence less than a criticalangle θ_(c) relative to the normal; first layer means including an airgap beyond said second surface relative to said base layer and said baselayer first surface for enabling light to enter said layer means fromsaid base layer when the light in said base layer achieves the angle ofincidence less than the critical angle θ_(c) characteristic of theinterface between the base layer and the layer means; light redirectingmeans for operating on light having passed through said base layer andfor outputting the light toward the viewer; and second layer meansdisposed in a layer position being a layer beyond said first surfacerelative to said base layer and being a layer beyond said lightredirecting means relative to said base layer, said second layer meansfor preferably transmitting light of a first polarization state comparedto a second polarization state and for preferably reflecting light ofthe second polarization state.
 54. The optical device as defined inclaim 53 wherein said second layer means is disposed beyond said firstsurface relative to said base layer and the optical device furtherincludes another light redirecting means disposed beyond both saidsecond layer means and said first surface relative to said base layer.55. The optical device as defined in claim 53 wherein said redirectingmeans comprises at least one of a reflective layer able to reflect lightand a transmissive layer able to modify the angular distribution oflight passing therethrough.
 56. The optical device as defined in claim53 wherein said second layer means is disposed beyond a first one ofsaid light redirecting means, relative to said base layer, with an airgap disposed between said base layer and said second layer means andanother air gap disposed between said second layer means and a secondone of said light redirecting means.
 57. An optical device for operatingon light from a source and for selectively outputting light to a viewer,comprising:a base layer having a wedge-shaped cross-sectional area andhaving an optical index of refraction n₁, and a first and a secondsurface converging to define at least one angle of inclination φ, saidbase layer further including a back surface spanning said first andsecond surfaces, and the light exiting said base layer when the lightbeing reflected therein decreases its angle relative to the normal to atleast one of said first and second surfaces and achieves an angle ofincidence less than a critical angle θ_(c) relative to the normal; firstlayer means including an air gap at least one of disposed beyond saidfirst surface relative to said base layer and beyond said bottom layersurface relative to said base layer and having an optical index ofrefraction n₂ for allowing transmission of light received from said baselayer; second layer means for preferably transmitting light of a firstpolarization state relative to a second polarization state, said secondlayer means disposed at least one of (a) disposed beyond said firstsurface and said first layer means relative to said base layer and (b)disposed beyond said second surface and said first layer means and saidsecond layer means further enabling reflection of at least part of thelight having the second polarization state; light redirecting means foroperating on the light reflected by said second layer means andredirecting it back toward said second layer means; and third layermeans for converting at least part of the light of the secondpolarization state to light of the first polarization state.
 58. Theoptical device as defined in claim 57 wherein said light redirectingmeans includes a reflective layer disposed underlying said secondsurface of said base layer and said second layer means overlying saidbase layer.
 59. The optical device as defined in claim 58 wherein saidthird layer means is disposed between said reflective layer and saidsecond layer means.
 60. The optical device as defined in claim 59wherein said third layer means is disposed above said first surfacealong with said second layer means.
 61. The optical device as defined inclaim 59 said third layer means is disposed below said second surfacealong with said reflective layer.
 62. The optical device as defined inclaim 57 wherein said second layer means comprises a plurality of layersof material having alternating high and low indices of refraction. 63.The optical device as defined in claim 62 wherein the indices ofrefraction n_(H) (high index) and n_(L) (low index) meet therequirement:

    tanθ.sub.H =n.sub.H /n.sub.L ;

    tanθ.sub.L =n.sub.L /n.sub.H

and

    n.sub.H.sup.2 n.sub.L.sup.2 ≅n.sub.H.sup.2 +n.sub.L.sup.2


64. The optical device as defined in claim 63 wherein the thickness ofeach layer of said plurality of layers is about one quarter thewavelength of light in said layers.
 65. The optical device as defined inclaim 63 wherein the thickness of each layer of said plurality of layersis at least twice the wavelength of light in said layers.
 66. Theoptical device as defined in claim 57 wherein said third layer means isdisposed between said second layer means and said light redirectingmeans.
 67. An optical device for operating on light from a source andfor selectively outputting light to a viewer, comprising:a base layerhaving a wedge-shaped cross-sectional area and having an optical indexof refraction n₁, and a first and a second surface converging to defineat least one angle of inclination φ, said base layer further including aback surface spanning said first and second surfaces, and the lightexiting said base layer when the light being reflected therein decreasesits angle relative to the normal to at least one of said first andsecond layer surfaces and achieves an angle of incidence less than acritical angle θ_(c) relative to the normal; layer means for preferablyoutputting light of a first polarization state compared to a secondpolarization state, said layer means including a first filter layermeans for filtering the light to pass preferably said light of firstpolarization state and said layer means further including convertingmeans for changing at least a part of said light of second polarizationstate to said light of first polarization state; a liquid crystaldisplay layer positioned to receive said light of first polarizationstate and output said light to the viewer; and light diffuser means fordiffusing light output from said layer means and said liquid crystaldisplay layer to broaden the light in a narrow angular distribution in aplane thereby outputting light to a viewer over a range of viewing anglewithout distorting image quality.
 68. The optical device as defined inclaim 67 wherein said layer means comprises at least one polarizationfilter layer and at least one light polarization converting layer. 69.The optical device as defined in claim 67 wherein said light diffusermeans comprises a parallel light diffuser.
 70. The optical device asdefined in claim 67 wherein said light diffuser means comprises a lightredirecting layer having a prismatic faceted surface.
 71. An opticaldevice for operating on light from a source and for selectivelyoutputting light to a viewer, comprising:a base layer having awedge-shaped cross-sectional area and having an optical index ofrefraction n₁, and a first and a second surface converging to define atleast one angle of inclination φ, said base layer further including aback surface spanning said first and second surfaces, and the lightexiting said base layer when the light being reflected therein decreasesits angle relative to the normal to at least one of said first andsecond layer surfaces and achieves an angle of incidence less than acritical angle θ_(c) relative to the normal; and first layer meansincluding a layer having index n₂ less than n₁ disposed beyond saidsecond surface relative to said base layer and for enabling light toenter said first layer means after output from said base layer when thelight in said base layer achieves the angle of incidence less than thecritical angle θ_(c) characteristic of an interface between the baselayer and said layer of index n₂ ; and a liquid crystal display layerpositioned to receive the light output from said first layer means andoutput said light to the viewer; and light diffuser means for diffusinglight output from said layer means and said liquid crystal display layerto broaden the light in a narrow angular distribution in a plane therebyoutputting light to a viewer over a range of viewing angle withoutdistorting image quality.
 72. The optical device as defined in claim 71further including at least one of a reflector layer, a light redirectinglayer, and a transverse diffuser disposed nearer said base layer thansaid liquid crystal display layer.
 73. An optical device for operatingon light from a source and for selectively outputting light to a viewer,comprising:a base layer having a wedge-shaped cross-sectional area andhaving an optical index of refraction n₁, and a first and a secondsurface converging to define at least one angle of inclination φ, saidbase layer further including a back surface spanning said first andsecond surfaces, and the light exiting said base layer when the lightbeing reflected therein decreases its angle relative to the normal to atleast one of said first and second layer surfaces and achieves an angleof incidence less than a critical angle θ_(c) relative to the normal;first layer means including a layer having index n₂ less than n₁disposed beyond said second surface relative to said base layer and forenabling light to enter said first layer means after output from saidbase layer when the light in said base layer achieves the angle ofincidence less than the critical angle θ_(c) characteristic of aninterface between the base layer and said layer of index n₂ ; secondlayer means for preferably outputting light of a first polarizationstate compared to a second polarization state, said second layer meansbeing disposed beyond said second surface relative to said base layerand said second layer means further able to reflect at least part of thelight having the second polarization state; light redirecting means forcontrolling angular range of light to be output from said device; and aliquid crystal display layer positioned to receive said light of saidfirst polarization state for output to the viewer.
 74. The opticaldevice as defined in claim 73 wherein said second layer means comprisesat least one of a polarization filter layer and a polarization converterlayer.
 75. An optical device for operating on light from a source andfor selectively outputting light to a viewer, comprising:a base layerhaving a wedge-shaped cross-sectional area and having an optical indexof refraction n₁, and a first and a second surface converging to defineat least one angle of inclination φ, said base layer further including aback surface spanning said first and second surfaces, and the lightexiting said base layer when the light being reflected therein decreasesits angle relative to the normal to at least one of said first andsecond surfaces and achieves an angle of incidence less than a criticalangle θ_(c) relative to the normal; layer means including an air gap atleast one of disposed beyond said first surface and beyond said secondsurface relative to said base layer and having an optical index ofrefraction n₂ for allowing transmission of light received from said baselayer; converting layer means for changing light of one polarizationstate to another polarization state, said converting layer meansdisposed at least one of overlying and underlying said base layer; and aliquid crystal display layer positioned to receive said light of saidfirst polarization for output to the viewer.
 76. The optical device asdefined in claim 75 further including light diffuser means for diffusinglight output from said layer means and said liquid crystal display layerto broaden the light in a selected narrow angular distribution.
 77. Theoptical device as defined in claim 75 wherein said layer means comprisesat least one polarization filter layer and at least one lightpolarization converting layer.
 78. The optical device as defined inclaim 75 wherein said light redirecting means comprises at least one ofa light redirecting layer disposed above or below said base layer and(2) a light redirecting layer disposed adjacent said back surface. 79.An optical device for operating on light from a source and forselectively outputting light to a viewer, comprising:a base layer havingan optical index of refraction n₁, and a first surface and a secondsurface, said base layer further including a back surface spanning saidfirst and second surfaces, and the light exiting said base layer whenthe light being reflected between the first and second surfacesprogressively decreases its angle of incidence about 2φ with each cycleof reflection relative to the normal to at least one of said first andsecond layer surfaces and leaves the base layer upon achieving an angleof incidence less than a critical angle θ_(c) relative to the normal;light redirecting means for controlling angular range of light in saiddevice and to be output to the viewer from said device; and layer meansfor preferably outputting light of a first polarization state comparedto a second polarization state, said layer means including a filterlayer means for filtering the light to pass preferably said light offirst polarization state and said layer means further includingconverting means for changing said light of second polarization state tosaid light of first polarization state.
 80. The optical device asdefined in claim 79 further including a liquid crystal display layerpositioned to receive said light of first polarization state and outputsaid light to the viewer.
 81. The optical device as defined in claim 80further including light diffuser means for diffusing light output fromsaid layer means and said liquid crystal display layer to broaden thelight in a narrow angular distribution in at least one selected planethereby outputting light to a viewer over a range of viewing anglewithout distorting image quality.
 82. The optical device as defined inclaim 79 wherein the range of angle of the output light has a redirectedangle of output arising in part from the angle of output from the baselayer upon decreasing its angle of incidence below θ_(c).